changeable backdrop

Active backdrop is probably your friend:

* Button btnLeft clicked
+ Image( "Active Backdrop" ) = 1
	 Active Backdrop : Set image to 0

* Button btnRight clicked
+ Image( "Active Backdrop" ) = 0
	 Active Backdrop : Set image to 1

Please login to see this attachment.

Actually, Active Backdrop is never your friend xD (no offense of course, just want to clarify some things )

Active Backdrop is an old object that haven't got updated in ages, it didn't even get a proper HWA treatment, meaning on any display mode other than standard, like DX9 and DX11, the performance would be terrible, way worse than Actives

So it will be more limited and worse in performance than just using actives, at least in HWA as I just mentioned, its worth noting that actives got really optimized over the years, they came so far to the point where they are almost as fast (if not the same) as backdrops!

If you plan on changing a backdrop in any way, just use an active, other than that, if you still really need to only work with backdrops, the furthest you can get is moving backdrops by moving the layer that the backdrop(s) exists in..
Also, no, you cannot change the type of a backdrop at runtime

Linky raises excellent points that I was not fully aware of. In fact my first answer was going to be, don't worry about it just use frames on an Active.

I'm unsure if you're building your backdrops directly into the Frame Editor or if you have a tile loading process, but probably the most-straightforward process would be to have your backdrops on separate layers and hidden at runtime. Then, just make the corresponding layer to your global value visible using the Layer Object. But, depending on the amount of art, size of tiles (if sliced into tiles at all) and other factors can inflate your RAM usage.