Need Suggestions: Pay for Prints, free download of web size image

Hi, I've got a gallery where I want to charge the customer for prints, and other merchandise, but allow them to download the small, web-size version of the image free. I've been looking at all the combinations for downloading, settings in the gallery, prices in the pricelist, but cannot find a way to do this.

I have a watermark on the images, so using "right-click, save as" doesn't work (because it saves the watermark on the image).
I can't make the price on downloads $0 in the pricelist, because there is a $0.01 fee.
I can't show the Download button, because it allows the original size to be downloaded.

Any suggestions?