If you have a program like Paintshop Pro or Photoshop, you can create a transparent layer on top of an existing character texture, merge them down, and save as a different texture like "Billy Torso 002.jpg" so you don't overwrite the main image.  Then when you pull up the texture you can change it out in the materials room.

There are a couple of programs out there, such as DAZ's LIE (Layered Image Editor) that work in DS but require .png files and I can't seem to get my older copy of Paintshop Pro to actually save them as a transparent .png.

If you find a good tutorial, let us know, ok?