An e-ink panel only uses power when it changes. Tiny capsules of black and white pigment are pushed into place, and once they settle they stay there, so the picture on a sleeping reader is not being refreshed at all. Many readers show around sixteen levels of grey, and some let you pick your own image for this screen, while others only show the cover of the book you were reading.
Sixteen greys is the number to plan around. A smooth sky or a soft gradient gets split into visible steps, or broken into a fine dot pattern when the device dithers it, and a dusky photo can turn into a flat grey card. Pictures with clear outlines, large calm areas and a real gap between darkest and lightest tones survive much better: a single tree against fog, a staircase in hard light, a line of rooftops. Converting the image to greyscale yourself before copying it over shows you most of this in advance.
Size matters less than on a phone, but matching the panel still helps. Many six- and seven-inch readers sit around 300 pixels per inch in a tall frame, so a portrait image at the device's own resolution avoids a second round of scaling and the slight softness that comes with it. Faint ghosting from the previous page sometimes shows through after a sleep, and a full refresh clears it. On a screen that holds one picture for days at a time, the choice tends to come down to whether it still looks settled on the second morning.
