I have continuity (ish) in all other email clients (including hotmail), using gmail however is a different story.
An image which is 72px high is been scaled to 7000px high for an unknown reason, code:
<img src="headerbg.jpg" style="width:100%;height:70px;" />
Are there any common errors for gmail?
Also, when inspecting the element I see this (which as you can see most of it i am not putting in):
<img src="http://gallery.mailchimp.com/663dc47a790bab652488be03b/images/
headerbg.3.jpg"
style="width:100%;min-height:70px;border:0;
line-height:100%;outline:none;text-decoration:none">
Your image has a very short width. When you set its width to 100%, you will get a very tall image, as the browser is scaling it accordingly.
Show us your raw HTML of your e-mail. I suspect that MailChimp is actually converting your height to
min-height.