I do it myself with my pics, from time to time, and the criteria I use in deciding whether to delete or not are number of comments and number of viewings. Now, if the site feels the need to reduce the disk space, or simply keeps things tidy, one possibility would be to introduce a "keeper" check button for the viewers. Not a rating, but simply a way of saying that in your opinion that particular image deserves a long(er) life in the forum. Sort of "survival of the fittest".
But roughly, yes, I agree that some clean-up would be beneficial, especially with more and more people joining.
Cheers, Marcelo