Welcome to TVPaint Community forums !

From what I've gathered, for the background, use an image layer.

Each time I add a frame in my anim layers, I have to keep extending the image layer -- is there a way to add anim frames without having to keep extending my background?

Do you mean this function:
"Hold" (after clicking on the triangle on the right of the image/background layer:)

Thank you!

I can't believe I never saw that. That's exactly what I was asking about.

you're welcome!