name of the background image

[selmiak]

how can I find out about the filename and filetype (png or jpg) of an imported background image of a room?

[Gilbert]

(Correct me if I am wrong) you cannot. These information are lost after import.
The background images aren't stored as-is like the original file. Instead they're recompressed (not using lossy compression like JPEG so it's safe) with AGS's own scheme to be embedded in a room file. So it's not a wise thing to do in importing JPEG images as background as this won't help in saving disk space.

[selmiak]

meh, too bad. But good to know, so the final BG will be png then

[Gilbert]

Yeah. It's recommended to use non-lossily compressed images for import. PNG is one perfect choice.