I'm not the best SMBX graphics editor ever, but I'm pretty sure that editing the mask images will change SOMETHING. I did everything right, I put the image into the folder called 1-4 and save the level, but nothing happens. Just so you know, all I did was turn background-11m 90 degrees.
Background-11m is in the folder. I didn't change background-11 so I didn't put it in at first. I tried it now, but it didn't work. All I get is a 4 by 4 black square.
Little questions: are you sure that the level is called exactly like the folder and that it's in the same directory of the folder? And are you sure that the graphics you used are .gif images? These might be stupid but they might also be your problem.
Can you try to show both in-game screenshot with background-11 being in the same folder as your modified background-11m without removing background-11 before taking screenshots?
Oops. Hehe. It turns out I AM stupid, and I WAS supposed to put background-11 in the folder too, but when I tried it before, I selected the wrong one in the game. (Red is what I selected, green is what I should have). Sorry...