If your pictures look bad on the published app, then it is either (1) you uploaded too small of images... we recommend 1024 pixels on the long side, or (2) it could be a bug.
If it is the case that your images look good, but not great to your eye, it probably has to do with the optimization of the apps. MyPhotoApp tries to optimize app loading speed and image quality ... with the slight edge going to speed. While this works well for the vast majority of MyPhotoApp users, there are a set of users with a discriminating eye that would prefer the utmost quality as opposed to speed. On the builder tab, in the publish options button, there is an option to optimize on quality over speed. Check that box and see if the image quality is better.