I use Microangelo Studio for creating/modifying icons and cursors. It has a button on the toolbar for adding a new image to an icon, and you choose among the images in an icon when you want to modify it. When you add a new image to an icon, the standard choices are 48x48, 32x32, and 16x16, or you can create a custom size.
I know I've seen this ability in other icon creation/edit utilities, but don't remember the names. They should be easy to find, though. Probably the freeware sites have some icon editors with this ability.
<edit>
Here are some how-to instructions in a couple of icon editors:
http://www.pcmag.com/article2/0,1895,1158580,00.asp
http://www.rw-designer.com/composed-icons
I know I've seen this ability in other icon creation/edit utilities, but don't remember the names. They should be easy to find, though. Probably the freeware sites have some icon editors with this ability.
<edit>
Here are some how-to instructions in a couple of icon editors:
http://www.pcmag.com/article2/0,1895,1158580,00.asp
http://www.rw-designer.com/composed-icons