Where to find/buy graphics to use in apps?

So I'm building my first app (a hockey app), and in a few hockey apps I've downloaded, I realized that some of the same graphics are used. For example, there is an identical ice skate that two unrelated apps are using. I've searched for where to find or buy the rights to this graphic (both for retina @2x and non-retina displays), among other graphics, and have come up short. I'm also wondering how to use photographs in the background--do I just use the image as a png or jpg? Thank you.