My book on memory improvement presents much anecdotal evidence that people with outstanding memories use mental images of what they are trying to remember. Now, a formal scientific study validates the conclusion that ordinary humans have astounding memory capacity for visual (but not auditory) memories.
In this study, young adults (20-35 yrs) were shown a succession of object images, one every three seconds. They were told to remember as much as they could. After about each block of about 300 images, they were given a 5-minute rest break. After 10 such blocks (total images seen = 2,500; total time about 5.5 hours), they were tested with probe images and asked for each one if it had been seen before. Probe object images were paired in three ways: objects that were in a different category, the same category, or the same object but in a different state or pose. Performance accuracy was remarkably high for all conditions, respectively 92%, 88%, and 87% accuracy. Remembering 2,500 images with this level of recongition accuracy is truly astounding.
As comparison, a related study by another research group showed that auditory memory was markedly inferior. When subjects listened to sound clips (conversation, animal sounds, music, etc.) and then asked to distinguish new from old clips, under all conditions performance was systematically inferior to visual-memory performance.
Apparently, everyone has a degree of photographic memory. Certainly, the odds of recognizing that you have seen something are very high, at least under conditions where the image is a simple object. The storage capacity is huge. Does this apply to complex images that contain multiple details? Who knows for sure? The details can serve as useful cues or could even become confusing distractors. It is also not clear, if the visual-image capacity is limited to recognition or whether it applies to generating a recall without an image probe.
Even so, it is a good bet that memory performance will be optimized if memory items are converted to mental images.
Sources: Brady, T. F. 2009. Visual long-term memoryh has a massive storage capacity for object details. Proc. Natl. Acad. Sci. USA. 106: 6008-6010.
Cohen, M. A. et al. 2009. Auditory recognition memory is inferior to visual recognition memory. Proc. Natl. Acad. Sci. USA. 106 (14): 6008-6010.
Remember, to get a full understanding of this post, you need the book, Thank You Brain for All You Remember.