As of Outlook 2007, MS ditched IE as its rendering engine to prevent the pre-loading of images, files and code that could be harmful to the end-users system. So, why would Outlook pre-load attachments in word that posed the same security risks as IE. That's kind of counter productive.
I may be misunderstanding you but what I gather is this:
New email delivered to inbox, Outlook pre-loaded the corrupt PNG, havoc ensued and you fixed it. Is that the basic flow of the situation? If that is the basic flow, depending on the version of Outlook you were using, it logically can't happen, unless you were using an older version of Outlook with IE as its render Engine.
There is a chance that we are also in the same book but on different pages, which quite possible with these topics.
Cheers,
Dave