I added checkboxes FRONT, FLIP, and FLOP under the tile selection
It's hard to tell, but there are now two layers of background tiles. BACK and FRONT.
BACK is the normal layer we've always had, but now you can select FRONT. This allows you to layer background tiles on top of each other (which helps if part is transparent). The Eraser will only erase the selected layer.

FLIP will mirror the selected tile horizontally. This should help reduce the number of images you need. For example if you have a tile for the left end of a platform, you can select FLIP to use it for the right end of a platform too.

FLOP does the same as FLIP except vertically.