I'll just quote what was said on this topic before, as it comes up every few months anyway:

Originally posted by Amantalado:

The issue with doing this currently, is that the way the game handles unit and building graphics is that it that it takes them bundled up with other things.

The game takes an image file that contains the unit's graphics as well as the portrait and their collision radius all as part of the image file (as far as I can tell, it may include other attributes also bundled together with the ImageID).

To make it possible to edit their graphics, there'd have to be a way to split apart the bundled file and then edit the piece you want (in this case the unit model) and then package it back together again to be used in the game.

As it stands, the only kind of changes to unit art that can be done are those that can be done via game scripts, such as changing transparency or changing the model color. Hopefully this changes in the future, but for now I've yet to find a way to do this.

Oh, i just watched pics using Majesty Library tool. but there was only option to export images. I hoped someone made tool to also import new/ change curent. Sadly we must still wait :|