Hi David. I'm sorry that didn't work, and I'm new to responding in Marketo forums, but I know how it feels to try to crank out something on a Sunday night and not have much support to work through issues, so I'll do what I can to help.
Here's the basic structure that I've used that generally helps out with IE, and email viewing issues.
<a style="text-decoration: none; border: 0px;" href="[link here]"><img style="border: 0px;" src="[location of image here]" /></a>
If that doesn't work, maybe paste your code in a reply and I'll see if I can shed any light on the issue you're having. A Marketo expert I am not, but I've wrestled with display issues before.