We are using iStockphoto, but at the same time are using every chance to produce our own photos and images.
I use the following resources to obtain free to use images:
publicdomainpictures.net - a growing source of public domain pictures and graphics
dreamstime.com - it has a large selection of free to use images, I also have a paid account here and a single image can be inexpensive at lower resolutions
Both have quick, effective search facilities.
Sorry the URLs are not linkable, I'm new here.
also search for stockXcg or something like this on google. The amount of images they have is impressive.