When people buy images on E-Bay, for example, they usually want pen-and-ink or pencil drawings, high-quality prints, or paintings (acrylic, oil). They may also buy CDs containing large collections of computer-generated images, but that may not be a good return on your labor. I think E-Bay clients are most receptive to hard-copy materials, but they don't want something that could be easily printed on an ink-jet printer, nor do they want to buy intangibles such as isolated jpegs or tiff files as downloads. One way clients will buy computer-generated images as downloads is for website design, splash pages, flash animations or similar elements for a commercial site.