...while I don't think copying cels from 
screencaps for your own use is wrong, or even for 
commissions or selling on eBay...I DO THINK that 
copying from manga pictures or manga artbook 
pictures is wrong, as mangaka generally do not 
like things such as that happening, and I try to 
respect their wishes. CLAMP especially is known 
for requesting that no one posts their artbook or 
manga pictures or reuses them in any way (it's on 
their web page, in fact); they want fanworks to 
be original. 

-Misha