Drop Down Menus And Images - Select A Site From The List

The intention is that when you select a site from the list, the banner underneath changes. However, I can only get it to do this once, then it simply refuses to let you select anything else. This is the code I am using to change it:

function defineimage()
{
if (document.forms[0].gourl.value = "http://jeff.zhomg.com")
{
document.images.linkimage.src = '/images/exchange/jeff.gif'
}
else if (document.forms[0].gourl.value = "http://www.strangedrawingsartgallery.com")
{
[Code]...