You don't need Photoshop. Any art package that can handle TGA files should be sufficient. If you don't have something like Paint Shop Pro, Microsoft Picture It! or Corel Photo Paint, etc, then there's Paint.NET (not normal MS Paint) that I know some people use, or there's the open source GIMP that's almost as powerful as Photoshop and completely free.
Some apps (like Irfanview) may even be able to convert the image to TGA for you without using a full-blown app. I'm fairly sure some people in the community have mentioned using it for images taken from other location when they weren't drawing/painting their own.