this feature seems a little to advanced in my opinion
but you can already do this with a few lines of external code
(make a program that periodically renames all the files in a folder with incremental names), set classic shell to one of those files, and make the program restart classic shell)
Example:
lets say you have
1.bmp - file A Your current button
2.bmp - file B
3.bmp - file C
with classic shell set to show 1.bmp
after the first run, the files would be:
2.bmp - File A
3.bmp - File B
1.bmp - File C Your button after first run
and so on...
asfar as coding it, this one wont be random, but it will atleast cycle all the files in the folder (which i actually prefer over random), but you could always make it random