The reason for EggImageDropdown to fail to work on IE7 and IE8 was that I had the page calling Egg within an iframe
which didn't have the following DOCTYPE declaration. It must be set as follows:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">