Probably, the real problem you are noticing is that whenever there is some scaling, even if it is very slight (and maybe particularly then), you are going to lose quality. To some extent, it depends on the scaling algorithm used by the reader, but they'll probably use quick and resource-cheap algorithms rather than quality-preserving resource-demanding ones.
Anyway, if an image is 400 pixels wide and you scale it to 410 pixels or 390 pixels (which may look to you as "the same size" on screen), you are going to see artifacts, blurriness or whatever. If you display it at exactly 400 pixels, it should be perfect, or as good as it gets.
So, whenever possible, try to avoid scaling an image, and instead of using a fixed "width" use "max-width" and/or "min-width". In my books, I usually just set "max-width: 100%" in all images, to scale them down only if they don't fit in the screen width, otherwise they're displayed at their native resolution.