At first I was frustrated when I pasted a high resolution image on top of a lower resolution image, then tried to shrink the high one down to match the dimension of the lower one by stretching (shrinking) the handles in the corner of the image. As I would do this the high quality image became quite pixelated. I can't explain why it doesn't just keep it's crisp resolution, but a workaround that solves the problem is NOT to shrink the high quality image but instead increase the canvas size significantly (i.e. go from 800x600 to 8000x6000). Then, to make the two images match in dimension, simply drag the lower resolution image up to the dimensional size desired to match the other one. Another undesirable characteristic of paint.net is if you happen to drag the corner handles of a picture to make it a dimensionally smaller size, then bring it back out to the same dimension as it was originally, it will distort the crisp resolution of the image. This characteristic may be by design... but it will sure mess with your image quality if you happen to do it, and the only way get your crisp quality back again is to hit CTL-Z (undo), or re-insert the image.
