I often say, if Delicious Library came with a manual, the first page would say "Do not look in the images folder. It will only give you a headache."
Of course, that would immediately send everyone to the images folder and sales of aspirin would skyrocket.
The plain and small images folder contain covers for all the "similar" items and are necessary to the functioning of Delicious Library.
In upcoming versions, better graphics routines will lead to elimination of the current system of image folders, but in the meantime, I would avoid messing with the images or bad stuff might happen.