It could be the mods prefer not to interfere in image plagiarism as it's a grey area, so they rather avoid making a decision. It would be better to hear directly from a mod, if it's a ban-able offense.
Yeah that's what I was thinking too, they simply don't wanna open that pandora box or set any kind of precedent on image plagiarism, no matter how obvious it is, and no matter the evidence. After all, that's why I probably didn't get any feedback from forum staff on that case.
I wonder if it would end up the same way if someone copied image from someone on the forum, and claim to be his own.