Add old screenshot from F-Droid

In the F-Droid store screenshots can only be added or updated. So to get
rid of the old screenshots a 1x1px version is added. After the next
release to the F-Droid store they can be deleted. From that point on
existing screenshots should only be updated!

See: https://gitlab.com/fdroid/fdroidserver/-/issues/490

Signed-off-by: Tim Krüger <t@timkrueger.me>