Open Game Art is a large community of creators producing art for you to use in your game. The art is free, although there might be some licensing restrictions such as the requirement to credit the artists in your game's credits. The site also allows creators to upload whole packages, so you can find images that go together and give your game a consistent look and feel. If you're an artist yourself, you could also consider sharing some of your art for other people to use!
- Browse the free game assets on opengameart.org