In general, when I see an update or find a new 
page, I look only at the sections for series I've 
seen and enjoy; I selectively go for cels from 
such series.  Whenever possible, I go for cels 
from specific scenes I enjoy, but I very 
frequently get any cels I can of a character I 
really like. ^_^;;
If nothing from series I've seen really stand out 
to me, I'll usually browse around in other series 
I haven't seen, but know, or series I've never 
heard of.  I'm a terrible whim-shopper, and so I 
frequently get cels from shows I've not seen if 
the character/design/art/scene appeals to me.  I 
get *anything* I like. ^_^;; And as such, I'm 
frequently broke.  
I've noticed some people seem to look down on the 
practice of getting cels from series one hasn't 
seen, but I think it's the greatest..  I knew 
absolutely nothing about Violinist of Hamelin, 
but picked up a cel of Guitar because I really 
liked his design, and thought the art was nice.  
It prompted me to research the series, and while 
I enjoy the anime, the manga has become my 
absolute favourite.  And Guitar's become my all-
time favourite character. ^_^  I would never have 
found the series or the character if I hadn't 
picked up that first cel on a whim.  The same is 
true of Ryuu Knight and Sarutobi (which have 
become another favourite series & character, 
respectively.)
A lot of things go into the decision for whether 
or not I'll purchase a cel.  Mostly, I like the 
value of having something of a favourite 
character, scene, or series, but I'm big on 
asthetic value, as well.  And from past 
experiences, I think it's a practice I'll 
continue.