“Using digital cameras to grab textures from real life introduces a whole new set of problems. Let’s say you find a wonderfully filthy concrete wall in a dark parking garage, which would work perfectly in the gritty urban setting of your game, and want to take a picture. The problem is, that due to the lack of light (and a tripod), you need to use the flash.”
http://www.cgarena.com/freestuff/tutorials/photoshop/flashlights/lights.html