Set Background and Foreground

Background (Color)

The background option in Set Background is the simplest, and allows you to change the background color for a room. It also has an opacity slider, which will be used for some fancier VFX tricks coming up.

Background (Image) and Foreground

Each room can have a single background and foreground applied to it, but these can be changed by using another Set Background / Foreground event.
The minimum resolution of the editor is 352x198 pixels.
Each image gets a choice of five different image aspect ratios:
The following images all used this 44x45 pixel image.

Scale to Fill

Aspect Fit

Aspect Fill

Center

Tiled

Tiled repeats the image across the screen until it is completely filled up. The bottom left most pixel of the image is placed in the bottom left most pixel of the editor screen. The Tiled image setting also allows you to set a scroll speed for the image in pixels per second, which will wrap the image.

Set Background Transparency

What if you want to have more layers at once though? This is where set background transparency and room movements come into play.
First Steps by lugi and AdipemDragon uses 7 layers to create a parallax effect
Rooms can be layered on top of one another by using several Move Room events, like so:
Now the rooms are on top of one another, you just can't see past the first one
By using a Set Background Color event and setting the alpha value to 0 for each room, you can place foregrounds and backgrounds with transparent sections in each room and see each layer.