I would think because backgrounds are often used 
for multiple scenes and cels, and that they truly 
aren't as 'one-of-a-kind' as cels are, that most 
original painted BG owners probably wouldn't care 
that you're using them.  

Also, many cels often come with laser copy BGs, 
which means that the original BG is out there 
somewhere.  No one seems to mind all this, so I'm 
guessing that you'd be OK just putting a little 
blurb on your gallery to appease those with a bit 
of an attitude about their backgrounds ^_^

Just my thoughts... someone else may disagree...
--amanda