i've found that image quality on ADE is notoriously poor, and agree with Jellby that this is likely due to the downscaling algorithm that ADE uses. i found a thread hidden away on an Adobe forum suggesting that best results are achieved by using 300dpi PNGs. however, i don't find that there is really a noticeable improvement in the image quality, and of course it greatly increases the size of the ebook if you have more than a few images.
here's the
thread with a fairly long discussion about image quality on the Adobe forums
--edit
another way that i've found to maintain a slightly higher image quality in ADE is to set fixed image dimensions, but in most cases i find this not a useable solution because the image of course won't scale with a resize of the viewing area