Editing the End of Level and Screen Size backgrounds?

Looking for some advice for a light TC I'm doing, I'm trying to change the background for the screen size (so hitting -), as well as editing the end of level background image,

I tried to edit the end level screen but some artifacts and sprites remain in the screen, so sprites like buildings, tried looking through the sprites but couldn't find where those were, same with the screensize, I exported the graphics to png but never found it, here's a screenshot for reference.