Well, sawyer's said pretty much everything I'd 
say.  It's an especial problem of DB cels--the 
studio puts whatever looks good with a cel, 
regardless of whether or not they belong 
together.  Add this to the fact that there are 
about 500 eps in the whole DB series (if you 
count GT, which I suppose I'll have to here), and 
that makes a lot of eps to sit through to see if 
you've got the right bg.  

Of the few DB cels I have with bgs, there's only 
one I haven't found in the anime, but that's 
because I've been lucky in finding my cels in the 
show.  I really don't feel like watching through 
the whole Cell saga again just to see if it's 
right ^-~.  Of the ones I've seen, I think maybe 
one of them has the right bg--the rest are close, 
or look good, but are not correct.  

That's why I like Lost Universe so much--anything 
that comes with a bg usually comes with a genga 
(stamped with Kane's ever-so-kawaii face on it!) 
that has the bg drawn on it.  So you don't even 
have to go looking--if the drawing matches the 
bg, more or less, you win ^-^.