I am sure you have though of all pricing models, but perhaps a free version that is limited to generating 256x256 size images, rather than a time limited demo?
The demo isn't time limited. The demo puts watermarks on the images it generates and prevents you from saving.
A demo which could generate only 256x256 images wouldn't sell, because people would assume that's as high as it can go, and/or would want to know what the textures look like at high res. Nobody wants to make 256x256 textures anymore.