This option may be late for you, now that you have removed the older
images, but it may be of value the next time you need to do so (and it
edits the grub boot menu when it removes the old images).

The product is called ailurus. It may have come installed on 12.04 as I
have it on my desktop, but it is in the software center, so it can be
installed directly, if not already there.