The 'b' versions of these files have never been used. They were added in http://hg.mozilla.org/mozilla-central/rev/5a4262b1b29d.
Bernd / fantasai, should they be removed?
If they're indeed not linked from anywhere, I don't see any reason to keep them.
Created attachment 536191 [details] [diff] [review]
Remove unused GIFs
Deletes two unused GIFs from layout/reftests/table-background/