So, update on the tumblr page:

I created site that has just the same layout and the same address which can used by multiple users. Unfortunately converting the old one wasn't possible, but with the way tumble is structured getting everything on the new site is very easy. However because of daily limits, only the first three batches are already up, the other two and the new one will follow tomorrow.
Since the url is the same, likes and reblogs are tied to the post and not the site, and images independet of an account, almost nothing did change for other users. The only exception are subscribers, who probably have to subscribe again. But everything you did with the old site should still be working as it did before.

On the bright side, contributing is now significantly easier. All you need is to have a tumblr account and to give me the email-address with which you registered, so I can send you an invitation to the user-group. Then uploading works just like on your own page and if someone wants to commit to a more active role, I can also grant rights to edit posts, add tags, and publishing images submitted by outsiders.