@ssj:
modelling is altering the 3D mesh that makes up the unit, skinning is changing the texture layer that goes on that 3D mesh... to skin check out some of the photoshop tutorials in the Scriptorium since they are very similar to how GIMP works


@vuk:
GIMP can edit .tga and with a plug-in .dds image files... skins are .dds format and are hidden in the pack files which you have to unpack using vercengetorix's unpacker program... there are also several image files such as loading screens (that aren't packed) or unit cards (that are packed) that are just .tga files that you can edit

hope i could help and someone correct me if i'm wrong
GMM