This is my first attempt at re-skinning. I personally use the model to represent an earlier cannon model available with the gunpowder tech. I also tried to mod the button for it, but it's buggy and shows a "!" instead of the button for e.g. current city production.
It's included anyway (and help is appreciated). Unzip the files into the desired mod or custom assets folder under
bombard.zip: art/units
and
bombard_button.zip: art/interface/buttons/units
(See Sovarns tutorial on how to add.)
Update: My button is probably all fine. I think the game is a little bit buggy when it comes to handling custom buttons though. E.g. the "Lost Units" mod has exactly the same problem (on my computer at least).
Update: In-game screenshot added.

bombard.zip: art/units
and
bombard_button.zip: art/interface/buttons/units
(See Sovarns tutorial on how to add.)
Update: My button is probably all fine. I think the game is a little bit buggy when it comes to handling custom buttons though. E.g. the "Lost Units" mod has exactly the same problem (on my computer at least).
Update: In-game screenshot added.