Certain subjects do not work for royalty-free images. Our clients license images to use them in all kinds of commercial and advertising applications. Because of this, our images cannot contain subjects which are subject to any kind of copyright or trademark protection.
The iStock Technical Wiki keeps track of many known copyright and trademarked subjects. The list is not exhaustive, but it is a good reference for knowing what you can and can't upload to iStockphoto.
4.0 Quality Standards
6.0 Title, Description & Keywords
8.0 Model & Property Releases