It might be that your images are not in a 3/4 ratio in their original formats. (When you reduce 600px X 800px too the lowest whole number it equals an aspect ratio of 3 to 4.) For example, if you have an image that is 600px X 700px and you tell the reader to change it to 590px X 780px or to 100% X 100%, the result will be that the image is distorted along the long axis. I would try to maintain the original proportions (aspect ratio) of the images to prevent distortion.

Another problem occurs if an image is considerably smaller than the 600px X 800px screen and then enlarged to fit it. In this case you can get a lot of pixelization--fuzziness. For example, if you enlarge a 300px X 400px image to 600px X 800px, it might look fuzzy, especially if the image contains lines or text. Quite often publishers use the smallest sized images they can to keep the ePub file size from getting too large. If those images are too small, there isn't much you can do to make them display well at fullscreen size.