One thing to remember with these small e-Ink screens is that its very slow compared to OLEDs, TFTs, or even 'memory displays'. It will take may seconds to fully erase and replace an image
There's also a recommended limit on refeshing - you shouldn't refresh or change the display more than every 3 minutes (180 seconds).
You don't have to refresh often, but with tri-color displays, the larger red ink dots will slowly rise, turning the display pinkish instead of white background. To keep the background color clear and pale, refresh once a day