Unless it's required by the GM, normally I won't bother with finding pictures and prefer sticking to text description. I feel that no matter how hard I try, none of the images I can find out there is able to depict the exact appearance of my characters - unless I draw the characters myself.

However, when I have to find one, it really depends on what I have in mind for the character - sometimes I get one before I start working on the CS, sometimes it's during the process of filling up the application, sometimes it's the last step after I pretty much finalized the sheet.