In the time since I wrote my script which downloads all of a user's TwitPic posts (text included) and uploads them as Posterous posts, Posterous has come out with their own import tool.
However, as noted in their blog post, TwitPic is currently blocking Posterous' servers, so someone came along and tried to use my script. It turned out that the TwitPic site had been updated, and my script no longer worked.
Well, I updated the script so that it works again. The script can be found here.