The Vargas and Elvgren pin up girl images are very copyrighted....how is it that they can be on this site and what precautions need to be outlined about using copyrighted images? Do you do a search for public domain images?
The short answer is that Fanpop provides an "image credit" field which every user should use to make it clear that no ownership or rights to the images are implied by being posted here: the images still belong to <these people> (what you insert in the image credit field).
Do a search of Fanpop (or just participate in a forum long enough) and you will quickly find dozens of pages of people discussing how to properly use the image credit field. One fairly straightforward method is described in this article: link