To submit new artwork, make sure your sidebar is shown, and click on Artwork in the Add Content section.
Give the image a name, and link to the original if you can (or attribute the original artist). These fields are not required, but they're a courtesy, and we'd like to keep them up as often as possible out of respect to those taking the time to make the art.
Use the uploader to attach your image file. Be aware that it may be cropped for size.
Once the image looks the way you'd like, set the status to Approved at the bottom, and you're all set! The artwork will now appear in your list to be selected.