Unit graphics as in sprites? I don't do too much of that, so I can't tell you specifics of what to do in a step by step guide... but I can tell you that you'll be needing the tool readBIF. I'm pretty sure it's somewhere in the downloads section.

What it does? Well it does what it's name suggests - it opens, reads and to an extent edits the .BIF files. BIFs are the graphics files that house all the unit's that you see on the battle-map. As I said, it is limited in it's use to edit them, but you can extract the frames (12 frames) as .bmp files and edit them using say Photoshop or GIMP. But other than that, I cannot tell you more.

Hope that helps.