I made a test today, how good WinRAR can compress a folder with several times the same picture in it. For that I just put one picture with 300 kB into a folder and copied it there 11 times, so that I had 12 times the same picture, only with different names. I expacted WinRAR to realise this fact and to get a compressed archive with less then the 300 kB of on picture. But the compressed folder was not by much smaller than the normal folder, namely 2.9 MB instead of 3.6 MB. The way I thought WinRAR would work, is by analysing returning patterns in the data and using them as an entry in its "dictionary". By that it would just have to save the picture once in this dictionary and then refer to this entry for each picture, only with a different header (e.g. for the filename).
Why does it not work like that? I mean, there can't be significant differences in the multiple files?