No problem is not in scan rules.
+*.png filter already will include all png file even from external servers, so
no need for trakker's suggestion

Problem is in get separate files. If you wanna get images using get separate
files option, then you need to manually find and copy all urls of all images,
because that is what it just does. It won't scan urls you put into main
window.

Use download web site method, which is default, to scan such urls. If you
don't like dir structure but want to keep all images in one folder, then use
different method in Build tab in options instead of site structure (default)