I actually use photographs I take to create my backgrounds! It helps that I live in the city my comic takes place in! I'll play around a lot with the colors/levels/contrast to make it look more stylized, and go through a process of adding a variety of filters (such as posterize, cutout, poster edges, etc.). Then I'll touch it up by painting over spots I think could look better. That way I can get super detailed and accurate cityscapes with minimal effort.
If it's something other than a city scape, then I'll use Sketchup to create the background. Any interiors, buildings that aren't location specific, etc. are all done that way.
If I were to do all this by hand, I would never have a comic to publish!