There is actually no requirement to follow a robots.txt instructions, though the vast majority of the robots/spiders that belong to the best known search engines do obey them. Those that go round harvesting email addresses to push Viagra and what not are an entirely different thing.
The problem is the thumbnails and the sources for the images in the Gallery are not stored on TV Forum, they're on third party hosting sites such as Metropol's upload site (but not exclusively), so it won't change the fact that Google Images has them cached even if the Gallery itself is blocked and Google could just go back to the original location of the posted images.
It may be better to do something like this to add, say, "this is a user mock" to the images, though how much load this would generate on the server I can't be sure as I think reading the documentation it would need to effectively host the images here at tvforum which could get cumbersome:
But of course if that happened, then it would be easier to just block that folder of the uploads from Google Images. So I don't think there is one straight forward solution here.