After tinkering with this I realized that the Echo Show does not display a sequence of pictures when you select an Amazon Prime Photo Album as its source. Instead, it chooses one photo and shows that continuously on the home screen. (I find this behavior counter-intuitive. It should be better documented.)
You can, however, initiate a slide show with the voice command “Alexa, show my album [album name].”
With this command you get to see all your photos but lose the time and temperature and various bulletins of the home screen and you cannot control the tempo of the slide show.
Amazon should consider making the slide show more flexible. I'd like to control its pacing and add date, time and temperature to the display.