I'm just getting into this myself. According to SitePoint's book on this, they suggest a width of 800px (like the old monitor standard). When people read these e-mails in their programs such as Windows Mail, it's a much smaller window than a regular 1024 x 768 monitor.
I wouldn't -- since I have all my servers set to auto-send HTML e-mails to the users trash... like a lot of hosts.
HTML e-mail pretty much just screams "Hey look, I'm SPAM" - so I wouldn't be wasting my time (or bandwidth) on them in the first place.
And something else I'm finding out (I'm pretty new at this, too) even though the likes of Campaign monitor.com will test your HTML e-mail in various browsers, I'm still getting replies back from recipients, "your last e-mail was blank. There was nothing in it." Back to the drawing board for me!
as a graphic designer I think a single picture is worth a million words and that although the take a while to load proper use of them is good; in fact, necessary sometimes.
However, in the case of email, I am going to have to slightly agree with death shadow and say that email is better off in text alone. Slightly because DeathShadow seemed, with all due respect, quite firm on his position regarding graphic email.
I just realised how disgusting, sorry if thats harsh, email with flash or animated GIFs would be.
Anywho, if it i needs images or styling it should be a webpage or a pdf not an email and that what I think.