FYI  From Admin:

"We have allowed members to create their own brand of thumbnails to use on their nude/violent images, as long as they only use the same one throughout their gallery on their nude/violent images.  As long as there is no violence or nudity on it, and no 'description' on said thumbnail..like 'nudity' 'beware nudity inside' 'image contains violence' etc.  It cannot contain any text or look like a censored thumbnail."

TOS states:

"If you fail to upload a thumb, our system automatically generates a thumb for you, which is a miniature of your entire image. If your image contains nudity or violence, you must not use this feature. You must upload your own thumbnail which excludes the nudity/violence and is within the thumbnail size limits (minimum 100 x 100, maximum 200 x 200, max file size 15kb."

http://www.renderosity.com/news.php?viewStory=13472

If you are in question if your thumbnail is within the TOS, just contact  a staff member.  We are always glad to help you.