Each menu link has a corresponding Itemid.

This is what I did to change the banner based on the menu link clicked:
global $Itemi;

switch($Itemid) {
case "1": $ja_header_img = 'header1.jpg'; break;
case "53": $ja_header_img = 'header2.jpg'; break;
case "75": $ja_header_img = 'header2.jpg'; break;
case "76": $ja_header_img = 'header2.jpg'; break;
case "77": $ja_header_img = 'header2.jpg'; break;
case "78": $ja_header_img = 'header3.jpg'; break;
case "79": $ja_header_img = 'header3.jpg'; break;
case "80": $ja_header_img = 'header3.jpg'; break;
case "56": $ja_header_img = 'header4.jpg'; break;
default:
{
//do default action if none of the item ID matches
}

Maybe you can adapt this to your situation.