I like to use Pixabay.com for finding pictures. Most (If not all) pictures there are free to use and you don't even have to credit the creator so they're good to use in editing. I know a strip of grass isn't recognizable from an edited Sims picture but I'd still feel bad if I stole a picture without rights. So that's why I don't like just choosing random images off Google.
Flickr.com is another good choice, although you need to make sure the licence allows the picture to be distributed and modified, while in pixabay.com all photos are free to use.