Very nice! (never could find the source code of the original fmon)
You could also work with visual cues, since nickel is going to display the image after all. If it actually appears on screen there should be no doubt left that it wasn't just a background database update or some other script fiddling with your files...
For example if the PNG image had to contain some bright pink pixels you could check the framebuffer whether or not such pixels are present, similar to how
scanline detects sleep/shutdown messages in my screensaver mod.
Taking this idea further, maybe you could also put a menu / touchable buttons in the PNG itself.
eInk is black&white but the framebuffer is actually color (RGB565) so a lot could be done with color codes (and should survive image scaling) that do not necessarily alter the appearance on the eInk screen.