Unless the size of the epub file becomes unpleasantly large, I format the images for a 1200x1800, and then in the css file I give the individual image width as a percentage. Then the images are shown correctly whatever the resolution of the device.
If the epub file becomes too large, I reduces the image file sizes by:
- Reducing the dimensions.
- Reducing jpeg quality. For old photos and similar, 65% quality is sufficient.
- Reduce number of colours in png drawings. For black and white illustrations, 16 grayscale levels are sufficient