You are correct, the issue is that problemRandomize.pl wasn't setting the $main::problemSeed when it changed the seed (it just changed it in the problems random generator), and so the new seed wasn't propagating to image names, as you point out.
I've fixed the problemRandomize.pl file so if you get the current HEAD version, it should clear up the image problem.
Davide
WeBWorK Problems
rerandomization and dynamically generated graphs
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