The images are easier to remember than words that recall abstract concepts because, unlike the latter (which only make use of verbal encoding), the former has a double coding: verbal and imaginative . Words such as "table", "notebook" are encoded by our cognitive system, both at the verbal and imaginative level, while abstract words like "epistemology" or "notwithstanding" are mostly only recorded verbally.
Dual coding theory
In scientific psychology the dual coding theory is inspired by Allan Paivio studies: «Dual-coding theory posits that nonverbal and verbal information are stored separately in long term memory. Dual coding theory is complemented by the theory of Alan Baddeley, in which working memory is divided into a visuo-spatial sketchpad and a phonological loop. » The visuo-spatial sketchpad is the store that holds visual information for manipulation; whereas the phonological loop (or “articulatory loop”) deals with phonological information or sounds. «It consists of two parts: a short-term phonological store with auditory memory traces that are subject to rapid decay and an articulatory rehearsal component (sometimes called the articulatory loop) that can revive the memory traces)» .
To make tangible also the abstract concepts we must therefore create images (according to the associative criteria already set and other methodologies that we will show) that represent them to us. In other words: where a concept has a merely verbal connection in our mind, we will put an adequate and coherent image able to create a new imaginative link; a new bridge with the word that denotes the concept. In this way an abstract thought becomes more concrete. By doing so, we are going to structure two bonds (one verbal and one imaginative) that will make us more familiar with abstract concepts. The fundamental principle should therefore be to attribute the words to the images. It doesn’t always have to be so, but only when the words don’t arouse the images automatically in us. It is even better to associate images to other images, at least when it’s possible given that a linguistic component is nearly always present in our thinking. However, it is rightly so for a lot of reasons, the first of them is that our thought is a discursive thought, the language is the principal medium (connection) between abstract concepts and concrete images.
An abstract thought, to be understood and to be explained to the other, needs a lot of words that describe it in an understandable way. To the contrary an image is intelligible in itself. It doesn’t have to refer to another thing to be understandable and memorable; so a complex thought can be easily stored by images, as long as the link between image and concept is clear to anyone who establishes it. In this way the word epistemology can be easily stored through the image of a scientist (e.g. Einstein) reading a book on whose cover is written “philosophy”.
The representations also should always refer to as many sensory channels as possible (visual, auditory, kinesthetic, tactile, olfactory), so going to strengthen the double bond between image and word. So doing, we can obtain a greater number of connective elements with the concept. If I think about a rose and want to connect the word “love” to this flower, I will have to think about not merely the image of the rose, but also of the smell and velvety feeling I get when I caress its petals. Here, I have made two very simple examples (epistemology and love), perhaps trivial, but other kinds of connections (starting from much more complex examples) can be made using the same criteria.
Visualization, reification, eidetic memory and synaesthesia
The visualization of thoughts is very important. If we have to remember a concept we form an image of it, or something relating to it, in our minds. An image that exists only in our minds. But, that image is then seen as if it were actually before our eyes, as if it were actually present there in front of us. In the art of memory the reification (i.e. mentally transforming abstract concepts into concrete images and giving them life as if they were actually real) is a phenomenon very close to the production of eidetic fantasies. In psychology eidetism  is defined as a phenomenon that occurs especially among children when they see unreal things and characters that do not exist as if they were actually present.
Awareness distinguishes eidetic images from hallucinations. In the art of memory we have to use these kinds of fantasies to recall things and concepts. In the art of memory these fantasies must be created voluntarily, not produced spontaneously. We should always imagine things as if they were true. The mnemonist must be exercised to see their fantasies: if he should remember to "take the keys" when he leaves the house, he must strive to see them and feel their weight, he must give to his own mind the strength to perceive “nearly” really what, in truth, is only in his imagination. However, if the mnemonist has this strong ability to imagine things, he has also the power to change these images. The ability to modify the imaginary objects goes hand in hand with the capacity to represent them; so not only we can help our memory by creating mental images ad hoc, but we can also adapt these images more and more to the subjects, topics etc. we have to learn. So, in our memory an object can have qualities and characteristics that in reality it doesn’t have. In the first instance the mnemonist must imagine how many more perceptual features an object can have. Then, he must be able to change these characteristics to the needs that emerge from the analysis of the material that he has to learn.
All of this can lead the mnemonist to use imaginary objects that have different and sometimes inappropriate characteristics compared to the real objects from which he was inspired. When this happens an important role is played by synaesthesia understood as a figure of speech. It is a device where one sense is described in terms of another and so specific sensory qualities of objects are mixed and confused with other qualities. We can find a lot of examples in the ordinary language and in literature: “warm/cool colours”, “the smell of money”, “taste of revenge” and so on. Synaesthesia is also a neurological condition in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway . The artist of memory is definitely a visionary, but aware of it! In the natural mnemonist this awareness is very attenuated. On the other hand, synaesthesia is a fact, and is quite common, although in attenuated doses. Sereševskij, a subject studied by Lurjia, had an amazing ability to quickly create mental images, but he was not well aware of the synaesthetic function of his memory: «Presented with a tone pitched at 2,000 cycles per second and having an amplitude of 113 decibels, Sereševskij said: “It looks something like fireworks tinged with a pink-red hue. The strip of colour feels rough and unpleasant, and it has an ugly taste – rather like that of a briny pickle . . . You could hurt your hand on this.” [...] “Even numbers remind me of images. Take the number 1. This is a proud, well-built man; 2 is a high-spirited woman...”» .
In conclusion, we can say that our memory can be enhanced using the imagination. The images must be well defined (though not excessively, in order to avoid unnecessary overloading), but we must also modify them and give them characteristics that can link them to the concepts we want to store.
 The reference to the dual coding theory is merely illustrative of an elementary principle in the psychology of mnemonics; for a broader insight into the topic you can start by consulting the following works: A. Paivio, Imagery and verbal processes, New York, Holt, Rinehart, and Winston, 1971; A. Paivio, Mental representations: a dual coding approach, Oxford England, Oxford University Press, 1986.
 From Wikipedia encyclopaedia entry “Allan Paivio”, saved Saturday, July 30th, 2016.
 From Wikipedia encyclopaedia entry “Baddeley's model of working memory”, saved Saturday, July 30th, 2016.
 We must not confuse eidetism and eidetic memory. The latter is a photographic memory: «is an ability to vividly recall images from memory after only a few instances of exposure, with high precision for a brief time after exposure, without using a mnemonic device. Although the terms eidetic memory and photographic memory may be used interchangeably, they are also distinguished, with eidetic memory referring to the ability to view memories like photographs for a few minutes, and photographic memory referring to the ability to recall page or text numbers, or similar, in great detail. In the case of distinguishing the concepts, eidetic memory has been documented while photographic memory is a popular culture myth that has never been demonstrated to exist.» From Wikipedia encyclopedia entry Eidetic memory, saved Tuesday, August 2nd, 2016.
 Cytowic, Richard E. (2002). Synesthesia: A Union of the Senses (2nd edition). Cambridge, Massachusetts: MIT Press.
 A. R. Luria, The Mind of a Mnemonist: A Little Book about a Vast Memory, Harvard university press, 1968, pp. 23, 30.