Thats a cool thought. It would be really neat if you used them for like... basic towers... and then used the regular skin for towers with upgraded defense... I mean, it'd be a hassle, but its just a thought.
Errr... I have never done this before, but I think that to do that, you must copy the mdx file of the tower and then rename it. Then, you use the mdx text editor to find the texture path it uses and change it to your texture. Then, you have to create a new unit, in the slks files, and change the model for the model you have just created.
some hints 4 getting screenshots from we:
-disable da grid (press g)
-disable da shadows (s)
-diable da lights (L), very important!
u can also disable boundary (b) n terraint (ctrl+t without terrain tool selected n puttin' maybe some kind of cool sky)