IE8 problem using Omega

Colorboxes don't display properly on IE8 using an Omega sub theme.

They seem to work on everything else, but on IE8 they appear and then shrink down to a thin white line in the middle of the page (See attachment)

I'm wondering if this is something to do with my having img set img in the css to max-width: 100%;

Strangely, it works fine on IE8 v8.0.60001.18.702 but version 8.0.7601.17514 doesnt display them. Hitting the 'compatability' button on the browser resolves the problem.

Can anyone help?

Thanks