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My best advice is to use Illustrator, Photoshop and Image Ready. The Adobe suite is good for creating graphics.
Another good alternative is the Macromedia Suite using Freehand and other offerings.
As for free stuff on the net, Google is the only answer I have. Try a search for "Royalty Free" graphics and the like. Royalty free simply means you do not have to pay for entire graphics collections. I happen to have 10 CD's of paid for graphics. It's all pictures and really nice to have on file. Purchasing a set like this allows you certain rights and is highly recommended.
Another alternative is to hit your local "Big Box" computer store and look in the software section for graphics sets. There are litterly tons to choose from, but I would research before you buy to see what others are saying about the packages.
Last but certainly not least, is to download XOOPS themes from those themers who do not mind people making modifications to their themes and just changing the elements on your own in your favorite graphics prgram to suit the colors you need. Masking, fading, blending, or cutting the corners or rounding the edges of some already done graphics can inspire you to create many different designs.
Sorry I couldn't help you more.
Good Luck