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I think your problem is because you don't really understand the point of Posterous. First of all, Posterous is just for fun. People post things on Posterous to share them with the people who follow them (think about the way people share links on Twitter, but Posterous allows you to post the content right to your blog) and then autopost them to their other social sites (Twitter, Tumblr, FaceBook, etc).
Second, when you post something on Posterous, there is an implied understanding that the people who are following you can "reblog" or "repost" what you've posted to share it with their followers. It's like, "oh cool, Ladymermaid posted an awesome picture, I'm going to reblog it so all of my followers can see it too." This functionality is similar to the reblog feature on Tumblr. In fact, in their Terms of Service, it states "Posterous respects your copyright. You retain full copyright of any original content that you send us. By posting to Posterous, you're granting us a license to distribute your content on this site", so that's why it's always ok to reblog things that other users post as long as you don't take them off of Posterous.
Third, on sites to share cool stuff (like Posterous or Tumblr), you're really only supposed to post things you have the right to use (like your own stuff or stuff in the public domain/Creative Commons) or excerpts or parts short enough that it falls under fair use. Their Terms of Service says that you shouldn't "Post content you do not have the right to transmit.", "Post content that infringes on trademarks or copyrights." or "Link to files on Posterous without a link to the original post."
You can check out their whole Terms of Service if you want. The point really is that Posterous is just for fun and to share cool things, when you posted your stuff on Posterous you gave them permission to reblog it anywhere on Posterous, and that not everything you do online has to be about making money. Stop and smell the roses and have a nice day