There is a program that very few people know about in Windows XP that allows
you to create, essentially, your own font/symbols. It can be found in the
C:\WINDOWS\system32 folder. The program is called eudcedit.exe, and it will
allow you to make your own characters. Once you create those characters,
you can reference back to them within excel. Limited however to the fact
that you can't use actual images or pictures if you will. You have a 50
pixel paint document to work with essentially--which means no color or other
more advanced editing features.

Here's a useful article on how to use it:

http://vlaurie.com/computers2/Articles/privchar.htm