blamax: like birrddog indicated, you need to find a image hosting server to upload your pictures to. You cannot upload directly to the pprune server.
It looks to me as if you tried to copy and paste the picture from your email account (fastmail.fm) into the preview/edit/reply window?! Since you are still logged in to your email account at that time, YOU can see the picture (so actually the link does refer to your picture, but it refers to your email server) but once you post it, it has no connection anymore - does that make sense?