is there a similar option (as like in mytvseries) that lets you a) download multiple images per movie and so they auto switch and b) limit these backdrops to either 1280x720 or 1920x1080?
You can only have one backdrop per movie right now although eventually this will change. You can limit the allowable size of backdrops under Advanced Settings though (button on the About page). If an image is larger than the max resolution of ra backdrop Moving Pictures will resize the image.
that will work for now (limiting resolution sizes), never knew that option was hidden away back there!
out of interest what happens if there are no 1920x1080 sized backdrops available?
If an image fails the minimum resolution requirement it will not be downloaded. If none of the available backdrops meet this requirement, then no backdrop will be downloaded. With default settings this is extremely rare though. If an image is too big (exceeds the maximum resolution requirements) it will simply be resized.