Digital photoframes are just a way to make yet another simple object cost a hundred quid and require a wall wart.

That would be precisely the point of making it static: it wouldn't require a wall wart, at least most of the time. (And ideally it should draw power from USB when changing the image, so that it wouldn't require one then either.)

Yes, OK, that'd be cool.

Sony already produce an eBook which works in this manner. It's monochrome only, but it only draws power to change the image. They use it as a book displaying device (http://www.sonystyle.com/is-bin/INTERSHOP.enfinity/eCS/Store/en/-/USD/SY_DisplayProductInformation-Start?ProductSKU=PRS500U2).