I finally managed to figure it out. I had to search for clipart instead of public domain art.
Larry Elmore released the "Character Clip Art & Color Customizing Studio" which is a fancy name for a colouring book. A good portion of the black and white line drawings were clipart and could be used and modified, as long as they weren't resold as clip art (and some of the more detailed artwork was for personal use only). It was also released as a d20 product, although that was probably just to get it noticed by his likely target audience of RPG fans.
It's nice that you can modify the art as well, so anyone with some artistic skill could make some authentic derivatives, even in full colour and with other content.