I've worked in two companies where customers will send us photos for warranty claims etc. If the photos are sent at a high resolution they'll be huge in Thunderbird and go off the right side of the page. You then have to open them in a separate program to view them in a fit-to-screen scenario. I think this is when the photos are sent as attachments.
If they were displayed at a width to fit in the viewport all my troubles would be over! 🙂