There is a way to legitimately offer it in your store, I believe: create it as a Photoshop .PSD file with transparencies in the areas where the base (copyrighted) Mike texture would show through, with instructions on how to overlay the Ninja texture to the base texture and then 'fuse' them together. People wouldn't even have to own Photoshop to do this -- an evaluation copy of Paint Shop Pro would also work (it handles .PSDs reasonably well). Of course, everyone who uses your Ninja texture would still have to legitimately own a copy of Mike's texture (or Steph's, I guess...).

I'm doing something like this right now with V3, creating a series of skin-tight textures that I'll eventually be making available for free on my website. Same procedure will apply -- download 'em, and use PS or PSP to meld them to the base V3 texture.

As for the aesthetics, it looks quite nice! Good job. And yes, I can see how it would be time-consuming, particularly if you aren't using one of the 3D editors to apply the texture correctly...

Good job!

Garth