Assuming you don't know how to code, I'll go ahead and just make that into a block of code you can just paste into the end of the cs file.
package vehicleHRL
{
function WeaponImage::onMount(%data,%player,%slot)
{
if(isobject(%player.getObjectMount()))
parent::onMount(%data,%player,%slot);
}
};
activatePackage(vehicleHRL);
All you have to do is paste that in and change the text of "weaponImage" to the name of the image datablock for the weapon. It would probably be called homingRLImage or something, you'll have to find it in the script.