Look up overlay and underlay in the refrence(F1 help menu)

You need to set up overlays and layers to do this. This would mean that you wont have to make a seperate icon for each possible combination (class and red tunic) Instead you would just create an icon for the red tunic as if there were a person behind it. then you would make it a overlay and set its layer to MOB_LAYER+1.

Then you need to make a variable under the objects (that you pick up) and call it something like "overlayimage". You set each overlay image to the objects overlay image (redtunic/overlayimage = /obj/overlays/redtunicimage.

Then when you call Equip(obj/O as obj in src), you would do this:
src.overlays += O.overlayimage

To unequip it you would do this:
src.overlays -= O.overlayimage

I hope this helps.... tell me if you want me to write out the entire coding for it.

PS:
I asked the same thing in the forum about 3-4 months ago.
^_^