If you have a file named album_foobar.png in the song folder, it will still be included in the .sng file, even with the --skipUnknown option. This is also the case for highways, backgrounds, and videos.
I'm not really sure if this is a bug or intended behavior, but it seems like a bug.
The bug seems to be here. The filename is checked with StartsWith instead of checking the whole filename up to the last .