Hi Ashley,
From the release notes from 9.20...
"Change: Email Template. Email tab. New option (on by default). "Embed Local Images". If on then the HTML part of the mail will be parsed before sending. If it contains any "local" images, then those images are automatically "embedded" into the email for you."
Your "working" email has a web-addressed image, the non-working one has a local image. That's somewhat strange because one with an existing web address should be included as before. I will investigate that a bit.
In the meantime try turning the template option off and see if that fixes it for you.
cheers
Bruce