This is some additional information about using licensed material for commercial sales. I spent a good amount of time in the last week trying to track down patterns/artwork that could be used in products that I could sell. I've found two sites that permit free commercial use: openclipart.org and vecteezy.com. At vecteezy, you have to check whether the author for the particular piece you want to use permits its use in a commercial product. You can also go to sites like shutterstock.com, but you could pay a (what I consider) hefty price to license a piece of art. For example, for two items, it's $199. It's something like $29 for two downloads it it's for personal use, but you really pay for the commercial use.
I also noted something interesting at vecteezy. There are, for example, portraits of Jimi Hendrix. The author's license says you can use it commercially, but I think the Hendrix estate might have a lot to say about that. I know they've gotten pissy with some commercial uses (I have a bottle of vodka with Jimi's picture on it that was pulled from the shelves after the family complained).
Maybe the wiki ought to have a section about where to get free art for personal and commercial use. I'm sure Will is going to chime in very shortly (if he hasn't already) to tell me that already exists.