With so many gmail users a lot of the posterous users may post from gmail.
So lots of the may want to post images inside their posts, but not at the end of the post, but inline the post.

There are two options, one of them is described in the official posterous.
http://blog.posterous.com/gmail-supports-inline-images-place-images-any

The other one is using imgur, I prefer it because it is independent of the interface being used to post, let's say Gmail, thunderbird, outlook, yahoo, hotmail or any other source.

To do it, just upload the image in imgur (really easy, just to http://imgur.com) and then place the link in your post.

Like this

Then when you send the email, posterous will load the image, and re-host it for you inline the post.

Here is a screenshot of how this post was seen while editing.

And all images are now inline.

Happy blogging