kernel images?
 

I am guessing compiled kernels are called images, but anyways: in menu.lst I have the option to boot a previous kernel version and my current one. Does that mean both are installed? If yes, how do I remove the old one?

Have a look in Synaptic - should be an option to remove. Make sure you also check "complete removal" tick-box.

Oh, and make sure you select the right one ...