For people who make DIY modules (like me), stock art can be expensive. I've used a lot of Rogue Genius's stuff, as they are pretty cheap to begin with and fairly decent quality. But now they've gone from $1.95 to 98 cents.
It's nice too, as you get a color and black & white version.
The main artist, Jacob Blackmon has covered a lot of the traditional D&D monsters, plus more outlandish, space, and modern stuff.
On the downside, their art comes in PDFs. I have never been able to copy and paste from them, they always appear weird in my paint program, no matter what I use.
However, I take them to Free online PDF Extractor and they come out okay