I'm just a user of the application as you are... Unfortunately I have no "superpowers".
Does the publishing error state any additional file information any missing file or something?
As soon as you put an image into iWeb the image gets copied into the Domain file so whether you took it from an external harddrive or not doesn't matter anymore.
If you are on a computer other than the original one you may not have the rights to edit an image or something but in that case you would get a permissions error and not a publishing error. You may want to perform a safe-reboot but that may not help or help I don't know.
Chapter 2.1 of my General troubleshoot FAQhttp://iwebfaq.org/site/iWeb_Troubleshoot.html
Or read my Publishing troubleshoot FAQ herehttp://iwebfaq.org/site/iWeb_Troubleshoot_1.html
If nothing works I really don't know.
I don't know what connecting file is... Connection data is the data/information you enter to publish the website. Since you publish to MobileMe your Username and your Password