This would work the best with my previous suggestion in #42 but it could also be useful for the resizing feature of the current version.

The main idea is to implement a option to choose, how a picture should fit in the given shape (square in current vers. any other with #42).
It should be a bit like the Windows wallpaper settings, where you can choose between the "zoom" (idk the official translation :P) and the "adjust" mode.