I have tested all opinion in Theme edit UI, such as fitstage, fitmargin, etc.
I have attached a screenshot, and you can see that about 20% of the right side is cut off:
P.S: This only happens for very large images. This image is 3504*2336. After I resized it to 1024*683, it goes well.
Unfortunately, this is a known issue with the flash player. Because of memory restrictions, it does not support images beyond 2000px(or something in that range). Images that extend that size either vertically or horizontally will simply be cut off.