As it is now, pressing F12 exports the entire interface as a PNG (scaled and everything).
I'm working on adding an option to export the current module's image as it is being presented on the editor. For example, if you're in the "NAM" module, editing "nametable 2" with "sprites: ON" and export a PNG image, it will save a 256x240 PNG file of nametable 2 with overlay sprites.