I was chatting with a stock photographer a while back. Apparently the most important thing is the tags that you give the photos. The best ones are abstract because you aren't competing with "hand" and "pen". If you can make your photo of a hand holding a pen fit "wistful" or "trenchant" then you have a better chance of it being seen. Making thousands of weird photos like a hammer/screwdriver/wand being held over a watermelon/plastic duck/brick by a clown/policeman/nurse and giving them abstract tags is the way to go.