I usually make my own. If you want to be 100% safe, just go take some photos with a camera. Like if its a business site and you just want a faint background of building windows or something, just go find a nice building and take a photo yourself, just don't make it obvious what building it is depending on the situation. (like making a bank site and using a picture of another bank LOL)
photoshop/other paint apps/3ds max/Autocad if you want to totally create something from scratch.
This is specially nice for nature related stuff, you can just go in the bush or something and you know that 100% whatever you take a picture of you won't get nailed for it.