document.getElementById("displayImage").getElementsByTagName("img")[0].src = "image2.png"
I seem to remember that for IE, you need to add a <meta> tag of some description, but I can't find which one.
If it helps:
Webpage error details
User Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.0;
Trident/4.0; SLCC1; .NET CLR 2.0.50727; .NET CLR 1.1.4322; .NET CLR
3.5.30729; .NET CLR 3.0.30729) Timestamp: Tue, 6 Sep 2011 12:23:00 UTC
Message: Object doesn't support this property or method
document.getElementById("case").src = "image2.png"? - punkrockbuddyholly