one way to post to posterous is using Posterous application downloadable from the market (http://market.android.com/details?id=com.Posterous). this application also lets you see and manage some aspects of your site. it has internal browser to view your writing or visit your autopost sites if you configure it. just open Posterous to create new post, add your text, attach some image or video, and put some relevant tags. it can be triggered from share button in other application, such image viewer. so you only need to pour some text. or, you might come from text or note application.

we can also use Gmail apps that comes, AFAIK, with every android. either manually composing new email or triggered by share button, just fill in the to field with post@posterous.com and the from field with whatever email address you register to post from. the rests are subject for the title, email body for the actual content, and attacment as you see fit. that's what Posterous apps does under the hood anyway.

one drawback in Posterous apps is it can't save draft. I hope it adds support in some future release. in Gmail apps, draft is a standard integrated feature. another drawback is image attachment. if you come from an image viewer with share action, the image is not attached automatically in the posting form. you need to attach manually. so I hope this one gets fixed as well.

enjoy!