Stock images can be used for inspiration, cut up and Photoshopped for other purposes like title screens etc, or used for reference when drawing or modelling. I use them often for the latter, because it helps when you are trying to draw something you aren't all that familiar with on a detailed level.
Anyway, here is my favourite stock image site: INMAGINE
Here is a totally free site: FREE IMAGES. There aren't as many as Inmagine, but they are unmarked med-res photos... approx 3000 in total in various categories.
Last but not least we have EVERY STOCK PHOTO which is a free stock image search engine.