Ten Things About Images and Graphics
Freeware caution: always scan free downloads of anything for bugs and other threats before dumping the programs into your hard drive.
If you're looking for images that contain the same colors as one of your own, you can upload your pic and get an entire page of them with Idée Labs' BYO Image Search Lab.
FotoSketcher is "a 100% free program which can help you convert your digital photos into art, automatically. If you want to turn a portrait, the photograph of your house or a beautiful landscape into a painting, a sketch or a drawing then look no further, FotoSketcher will do the job in just a few seconds" (OS: Win 2000/XP/2003/Vista)
Mark Tyler's mtPaint is a paint program that "uses the GTK+ toolkit (version 1 or 2) and runs on PC's via the GNU/Linux or Windows operating systems. Due to its efficient design it can run on older PC hardware (e.g. a 200MHz CPU and 16MB of free RAM.)" Also operates in several languages other than English (OS: Win 9x/ME/NT/2K/XP/2K3; Linux)
Multicolor browses for images on Flickr that match a color you pick out from their grid.
If you'd like to try uploading an image to a sketch generator (like an online version of FotoSketcher), stop by Sporkforge.com's Sketch Generator.
Retrievr will also find Flickr images that match something you sketch in their little paint window or upload as an image.
Sqirlz Morph multi-morphing freeware allows you to "mark features on several images, and the program mixes the images so that each feature on one image moves smoothly to the corresponding feature on the other images - a technique widely used in the film industry" (OS: Win ME/2000/XP/Vista)
Someone (you know who you are) asked if I knew of a search engine that would match an uploaded image to something on the web (helpful if you want to check and see if your cover art had been used before) -- you might try the reverse image search engine Tinyeye.
Also from Idée Labs, the Visual Search Lab Alamy Set allows you to search by tag and pixel color.
In need of public domain clipart? Browse the 30,500 images over at WPClipart.