I'll admit I do not know the solution to view imagines in html email messages with the default client in diablo. There are some alternatives:

One is Claws-email:
http://www.claws-mail.org/maemo/

And install the Gtkhtml2Viewer plugin as well.

This displays html emails quite well but the rest of the app is less intuitive than the default email, so you may not like it.

I also regularly use my gmail and other web based email from a browser such as tear. I think ther is a gmail checker desktop app out there somewhere as well.

Cheers