Generally speaking, if you modify someone else image you may be able to claim it as your own. Where does renderosity stand on this, particularly when it comes to the galleries? If I find an image from anywhere and modify it, can I place it in the gallery?

Generally speaking it is NOT legal to modify something and claim it as your own (Copyright Infringement) without crediting the Author and still if anyone says anything about it or staff notices you better be able to show proof you have permission to use it or it will be pulled and possible Banishment for you, especially if it's a habit....unless it's of course it's in the Public Domain or stated for public use by the originator

Winter - I'm looking into this as we talked about in chat. :)

It depends. A lot of artists do manipulations and they use stock specifically made for that. For example they may use someone's background, a picture of a person, object, etc and combine them. But those images are sold/given away as "stock" and there are terms of use as well. Some may not be for commercial works or credit and a link to the creator's site is required and so on.
But as far as copying an image from the gallery here and modifying it, that it is not permissible as this is a gallery of finished work and not stock for others to use and the copyright rests with the creator of the image. I'd suggest you take a look at deviantart where many users upload free stock (with terms) for the use that you describe.