Currently, after using the green screen effect, the only built-in background option is a solid color. If you want to use an image instead, you have to manually place the image behind the video as a separate layer.

It would be really helpful to have the ability to set an image (or video) as the actual project “background” rather than just another layer. That way, when using features like Layers or repositioning clips, the background stays locked in place and moves consistently with the composition instead of acting like an independent object.