How to Add Edit
Simply place any .png image in the "backgrounds" folder in your mappack. To add multiple layers, add incrementing numbers starting with 1 to the end of each image name. The higher the number, the lower the layer, and the further away it is from the level. If there are any places where the background is transparent, the background color will show through it.
Backgrounds can be any size, but the normal screen size is 400x224.
Some older versions require numbers at the end of the title, even if it's only one layer.
Adding backgrounds works very differently in 1.6. The image name must be the same as the level—down to the sublevel—with background added to the end (i.e. For 1-1, you would name it "1-1background1.png"). Because of this, even if it's the same background, you still have to make a copy. You can also name it "background1.png" to replace the default portal background.