You can't (currently) save textures out of AC3D. Your best bet is to use a "Light-Mapper" to create a 2D light map and then texture the light map. I say that because the only real benefit to saving your texture after having introduced it into AC3D is to save the lighting information.
I recognize that light-map might be the correct term, since the term light-map usually refers to a texture used by renderers to generate lighting information.
Someone else could probably explain this better.
Gile[s] was suggested to me.
Edit: Textures are always a separate file. I don't know about procedural textures, but bitmap textures are.