It appears that the room editor modifies my graphic from 640x360 to 640x400 when I import it. However, when I try to load via background animation another 640x360 graphic, I get an error message saying the background animation is 640x360 (which it actually is) and doesn't match the room background. Consequently, I can't load any background animation graphics, even though every graphic I have is exactly the same size. This happens with both .bmp and .pcx. I've tried with game resolution at 640x480 and at 640x400, but the same thing happens.
If your game runs at high-res 640x400 mode, make sure you make the backgrounds at least of this size (usually by adding blank space on the bottom of it, where you for example will place a GUI there). Importing a background image (no matter it's the base background of the frames) which is smaller than the game's resolution into a room is not gauranteed to work.