Tumblrs are ridiculously easy to set up. I'm currently running one for English Wikisource (although I haven't done as much as I would like as I'm limited on time).
If you want a Commons tumblr, you might need Commons itself to buy into it first. At least, if you want to refer to it as a Commons tumblr; it would be the polite thing to do if you intend to act in their name. If it's UK-related (not necessarily WMUK, just generally local), it would be simpler.
Scope and morality aside, it is, as mentioned, very easy to get going. I could have set it up faster than composing this e-mail. (NB: I could get it started for you if you want but other people would need to maintain it -- ie. post images). Secondary tumblrblogs have "member" and "admin" statuses; the first can post, the second can grant member/admin powers and de-member people again. Admins can't be de-admined, they can only voluntarily quit; which is why you should probably be sure from the start about who is running it.
TTFN,