To answer your questions:

I have one cel framed. It is a cel with a 
matching background (my hotohori/boushin OVA cel 
if you wanted to look). 

I had my cel framed proffesionally AND by me - if 
that makes sense. I work in an art/frame shop so 
I worked on it as well as the professional 
framers that work with me. 

I picked the cel of hotohori/boushin for the 
first one framed because a)the part it came from 
was so touching and b) the image itself is 
completely closed. Meaning: Their entire bodies 
are within the cel. It kindof looks like a 
snapshot to me. I would suggest looking for a cel 
with composition and meaning to frame.
Unsure of composition? Well, hold the cel upside 
down and look at it. If it still looks "good" 
(balanced visually), then it has nice 
composition. 

Hope that helps
~Tara