Stock photo's are pictures of high quality you can use in your designs... for example:
http://www.sickdamien.nl/work/art/humanmainframe.jpg
This is an artwork done by a dutch artist. He used a 'stock photograph' of a woman and modified it till he got this...
But it isn't just humans, you can have stocks of everything imaginable... Trees, animals, walls... even pieces of broken glass...
Got the point now???