So the first version of this concept used a Raspberry Pi Zero and received the images by email. I liked how easy it was for anyone to send images by email, without needing to remember a link (although it needed a specific password in the subject line to be displayed). But the email was a googlemail account, which was a mess for me to set up without getting it blocked for accessing it automatically and so on. And I never fully trusted google not to change the service in a way that would break the system.
Also, the Pi was definitely overkill. It allowed a lot of nice onboard image processing options, but that really wasn't necessary, and the whole startup and graceful shutdown issues made the power supply a lot more complex. I used a nice 16bit greyscale e-paper display though, which looked really good and it had an 1872×1404 resolution, so it was actually quite hard to see that it wasn't a printed image. The contrast is better with the lower resolution displays I'm using now. Also, the high resolution display needed a bit more complicated driver, and was about double the price. I might go back to it in a future version.
My brother made a nice wooden frame for it, with a water jet cut and bent aluminium backing (still looking for the photos though). It looked quite nice but the inside was held together with blu-tac and tape, and was a mess of wires.