https://bugzilla.redhat.com/show_bug.cgi?id=2024917
--- Comment #3 from Zbigniew Jędrzejewski-Szmek zbyszek@in.waw.pl --- I haven't done an appdata file in a while… but I remember that generating something that would display properly in gnome-software wasn't easy ;)
'convert -geometry 64 input.bmp output.bmp' should work, though the result won't be very visually appealing.