If you want a free service (as Posterous was) there are only two options where you can migrate your Posterous to without writing all your posts one by one again:
So, if you want your blog back including images the only option you have is Obolog. There are no other free services in the net (or at least I didn't find other) where you can bring back to life your Posterous blog. If you have already migrated to wordpress go and see where your images are pointing to. You'll be disappointed because all the posts reference posterous server, and they will be shut down in a few days.
If you still wonder what Obolog is, you are just reading one right now. Harecoded is powered by Obolog and it has been running smoothly for several years so far.
If you want to try it all you have to do is to upload your ZIP backup file into the Posterous migration script. But remember, on April 30th if you don't have the ZIP everything will be gone for good.