Upload & Sell: On
Part of the distinction @ registered images vs. unregistered images is that the attorney's fees can be awarded in addition to the actual damages (monetary loss) and a punitive (punishment) award can be awarded also. The amount can be as high as $150,000 (iirc).
As to the workflow @ registration, you have roughly three months to register them. So, you don't have to concern yourself with registration in the course of your regular workflow. With a 3 month window (or 90 days, I forget which), that basically means you just need to submit your work for registration 4 or 5 times a year to always have all your work registered. A quick look, at one of the links seems to suggest that a bulk (calendar year) option might be available in addition to the three month window. I'm not current on any changes that have occurred in the last couple years ... but this is the gist of it.
Ed Greenberg and Carolyn Wright are a couple of attorneys that are well versed on the matter, with Ed having some materials (book /DVD) to further enlighten us on the subject.