The workflow for OTRS image submissions would be pretty much the same as if someone emailed them to you in your normal email client. They come as attachments, which the worker would have to remove to their local computer, and upload like normal. The OTRS system does have canned message replies, but they have to be chosen, and sent. There is no integration with wikipedia/commons of course.
It does have work queues, and the idea of people being assigned to emails, which is probably nice for this. Also, I haven't looked at the emails in this queue, but I can almost guarantee people aren't sending all the required licensing information in the first email, necessitating followups etc.
I don't have a really strong opinion either way, OTRS would probably be better for the followups, and queueing, but it might be nice to select a license from the a dropdown, select the attachments, and click upload also... :) If you were a little more strict in what needed to be in the email (licensing) maybe a toolserver system would work. Declined messages could say what they are missing, and they could just try again. OTRS is usually a little more hand-holding, and that may be a benefit in this situation to some people.