This page lists a variety of resources that you can use while creating graphics for your game. Some of these sites have images that you can use in your game for free while others can be used as examples and inspiration - please read any release notes before including someone else's graphics into your program!
These references show you how to create your own bitmap graphics.
Ari Feldman's Designing Arcade Computer Game Graphics was published back in 2000 but is now out of print. However, you can download a complete PDF of the book.
Many sprites are freely available for you to use in your games. They are typically in the form of a single "sprite sheet", which is a single PNG or BMP file with all the sprites arranged on them. To use these with Spritely, you'll need to redraw them within Spritely.
To find them, do an image search for "sprite sheet".
If you're planning on releasing your game, you cannot make use of these sprite sets since most/all of them were extracted from commercial game releases (or involve copyrighted characters). However, they're a great source of inspiration and allow you to see how the professionals create and animate their sprites.
The ShyGuy Kingdom has lots of sprite sheets from various games.
Video Game Maps has a number of background maps extracted from different games. This is a great reference when you're creating your own background maps.