To edit backgrounds, you need to edit three modifiers: background, bkg X pos, bkg Y pos. To place a background, you need to set the background modifier of all tiles to the same and set the X and Y pos modifiers according to the position of the tile inside the background. For example: if you want to place a tile that is 4 tiles wide and 2 tiles high:
X=0 Y=1 | X=1 Y=1 | X=2 Y=1 | X=3 Y=1 |
X=0 Y=0 | X=1 Y=0 | X=2 Y=0 | X=3 Y=0 |