Apologies up front for this probably being posted in the wrong place, but wasn't sure where to post it.

If a vendor inserts a a requirement/phrase/statement that either directly contradicts or adds to Renderosity's EULA, is this statement binding as the EULA is, or is it, from a legal perspective, considered a consideration on behalf of the artist. For example, if a vendor states within the Read Me, "Artist must provide accreditation for any commercially rendered images using this product..." or something to that nature (which I have seen in Read Me's), is that binding legally? Just looking for some clarification.