New here.
I took a look in knowledge base and didn't find the answer to this question, but if it lies somewhere, a simple link would suffice.
How are the background layers determined? Are they separate image files, or can AGS parse the layers of a Photoshop psd?
Neither.
You draw a separate picture that shows the layers in colors (e.g. make the entire front layer white, the middle layer red, the 3/4ths layer green, etc). It's called the walkbehind.