I place my pictures on Picasa Web Folders at a lower resolution than the originals to save bandwidth, and to keep people from stealing my originals. The latter reason is why the pictures also have a watermark on them.
When you purchase a picture I remove the water mark, and print the picture from the original high resolution file. Each purchased print also comes numbered and signed by my hand. They're not expensive, unless, you want an extremely large and framed print (we're talking feet, not inches).