Digital, huh?  Well, Adobe and Photoimpact both 
have a tool called clone-stamp, with which you 
select a point on the picture and then "copy" it 
over something else.  Works best, though, with 
solid color backgrounds, or something with a 
repeating pattern like grass, or a wall, etc.

And then there's that whole resolution/picture-
print-size thing to consider. Doing watercolors 
seems easier - except for the really detailed 
ones!  Which brings me to a question I've been 
thinking about backgrounds - painters out there: 
are production backgrounds done using watercolor 
with very little water or are they done with a 
more opaque water-based paint like goache? It's 
been a long time since I dabbled artistically, 
but the amount of detail in some backgrounds 
seems to require something more than highly 
transparent watercolors.  (Attached is the most 
expensive BG I ever got.  Just love this park 
scene!)