Ok, now I'm sure it's an .zip file, but when I try to start it, a pop-up says: "No image found on zip file" and when I press ok another comes up who says: Unsupported file type. It's just something wrong with the Emulator? But I can play .gba files on it..
you haven't answered our questions,
Can you open the ZIP file properly?
are you able to extract it?
perhaps its in some form of Compression [like deflate] which I think the emulator doesn't support]
or maybe the zip has a password
please try to EXTRACT the ZIP then play the GBA file on VBA directly
then ZIP again [if it works] with normal settings