Hi stefkant!
I checked your website on IE8 mode on Win7 and it does show up fine. Please try adding following code to Quick CSS in Enfold theme options under Styling tab
.logo img { width: auto; }
Also, you are using an older version of the theme. Please update and check if that helps http://kriesi.at/documentation/enfold/updating-your-theme-files/
Cheers!
Yigit