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.