Found another probem with the Post Office plugin for Movable Type: you can't include any kind of punctuation characters in the subject line of your email message/entry title, otherwise it will munge the image file name that goes along with it.
Granted, the Post Office plugin isn't to blame for this, since it won't rename the image at all; this was functionality I had to add in later. So, really, it's my own fault.
Well, it's fixed now. Weird chars are replaced with underscores.