You can search for royalty free resources on google.
Yeah, good luck finding enough off free stuff, so game wouldn't look as stolen product.
I have hard time doing this, because if i get a good image and then a bad image it is visibly too obvious. Also matching it all together so its fits the graphics is a pain.
That being said, set your standards low, put time intro the images as a artist would do and enjoy!
I also use gimp, its just so nice and easy to use.
Also most images could be used, just have to credit the maker for it.