I use Dreamstime (free and paid) and a site called photos8.com.
Always make sure you are not violating any copyrights or usage laws. Be sure the image can be used for commercial use.
And always give credit back to the artist, a link to the original location is fine. I always do this when I get free images.
I think you know all that already but I always remind people. HubPages will remove them if you violate any rules.
Have a great day!