When you shoot a non-editorial stock photo you have to be very careful to not have any copyrighted or trademarked objects in your photos. That means no logos on clothes, no easily identifiable copyrighted buildings.
I was going to write more about copyright but I noticed this great entry at Media Bakery Design that does a great job and includes a list of copyrighted work as well.
What this means in stock photography is that if there are any logos or other trademarked material in your photos you must edit them out before submission.
Note that if you plan to sell your photos for art (so NOT stock photography) that includes trademarked material that’s another story. To quote the World Intellectual Property Organization:
“Unlike copyright law, trademark law as such does not restrict the use of a trademark in a photograph. What it does forbid is the use of a trademark in a way that can cause confusion regarding the affiliation of the trademark owner to the image. If consumers are likely to mistakenly believe that the trademark owner sponsored a photograph, then there may be trademark infringement.”
Notice that this is just my understanding of it, and this is not intended to be legal advice, please do your own homework, laws do change…