Try going to Manage | containers and clear the "Generate and store thumbnails..." check box.
See if that makes a difference in the image quality.
I think FileMaker needs to add a way to selectively enable thumbnails so that they can be used for some containers and not others or so that a container field object can be formatted to selectively use or not use the thumbnail.
No, there is no difference. I don't think that the thumbnail is the problem: I have the same quality loss in pictures, that are part of my layouts – and in this case there will be no thumbnail, I suppose.