PS:
Looking at gameplay footage...
Yes, it would be great if the girls are fully clothed initially and in in-game pictures when using skill they are clothed, Then when upgrading them (leveling the girls, or evolving, or whatever you have there) they get gradually more and more revealing clothing until fully naked (at max lvl/evolution/whatever).

The pictures in battle would be the same poses, just different clothing - shouldnt be too hard for an experienced artist as the base picture is the same.

The player however, would be like --WoW what a sexy pose, i wanna see her do that naked--... and they will stick with the game, upgrade their girls, and throw money at you