Internet Explorer has some issues with some types of image sizing parameters, but from looking at the CSS for the blog header, I don't see anything that should cause distortion.
This is a wild "pie-in-the-sky" guess, but add this to the bottom of your custom CSS and then see if you can get someone with IE to take a look.
Also, wordpress no longer supports IE7 and earlier, so if the people are using 7 or 6, that is likely the issue and it would be good to encourage them to upgrade to a more modern browser. IE6 and 7 are being left behind by web development at an astonishing pace and web designers don't even try to stay compatible with them anymore since their market share is under 1% of total browser use.