I don't see the difference between Posterous and Tumblr. The latter also has an email address that you can send anything to and a great bookmarklet. Is it simply that Posterous sells themselves differently?
With Posterous, you can send us anything by email and we'll handle it. Multiple images, video, audio, other documents all in a single message. That message can also have full html and we'll publish it all.
We autopost to Twitter (tumblr does this now) but also to Flickr, Facebook, Tumblr, and all the major blog platforms. So you can email us once, we'll handle all the rich media, and then update all the websites you use.
We also have a great bookmarklet. You should try it out.