Hi,
I have had this same problem. I tried removing the SD card, or saving the photo on the phone. I took me about 15 minutes of working out which way to do it, but i changed it once. Now i have gone to change it again, and it keeps switching the the default picture and now i cannot revert back to the first lock screen i have. I can change it to some of the preloaded background pictures, but can't make my own the lock screen.
I saw your comment and tried to manually crop it, it worked when i zoomed all the way in on the picture, but obviously this is not very practical. I also tried the screenshot of the picture, and this did not work either. I did so by going to the picture and clicking 'set as lock screen', cropping then clicking the tick. Also tried going through to the lock screen in settings, and could not change it to a normal size without having to zoom on both of those methods :unhappysweat:
Anyone else know? It can't just be the SD Card.