Most clipart packages have legal info detailing the use of their images. Most allow commercial use of the image as long as your not using said image by itself and selling as a product. Essentially you cannot grab a peice of clip art, put it on a shirt and sell millions. You can add text around the image and make it into a design and sell.
Also, most only allow up to a certain amount the image can be used, like 500,000 copies. After that you have to pay them royalties, but your probably safe to use the image as you wish. If you put a peice of clipart on a shirt you cannot own the copyright. the clipart company owns it.
Make sure you read the copyright notices and the company use regulations. Generally the clipart that comes with a program is not for resale purposes - so you cannot use it on a product where you will be making a profit.
If you're looking for clipart you may want to go to Office Depot or Staples and check out the cd's they have available. There are tons of images that are royalty free so you can use them as you like. You may also want to visit http://clipart.com
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