I'm probably broadcasting old news... but I found a link at NVIDIA for working with DDS files which are the texture files used by Civ4 units... Here it is (http://developer.nvidia.com/object/nv_texture_tools.html)

One of the more exciting things is a plug-in for Adobe Photoshop... works nicely. I used it to make the image below.

I've posted a pic here of the Blue Coat from the American Revolution mod here (I've put a "watermark" over it in case I'm not allowed to release unmodified skins)

Whoops! Seems somebody did post a link to this already.... :o