Repainting w/ Photoshop 7

Hey,
I am completely new to the whole repaint thing, and I was wondering if I could begin now. I am sorta aware of how to edit textures, but then again, that is Halo, and that had a pretty straight-forward program, or HaloMapTools. Anyways, I have Photoshop 7 and i used that for the textures. However, I thought that it would be easy enough to just open up the texture files and mess with them using Photoshop, and I am apparently wrong. I get an error message saying that it could not open.
My point is, is there a tutorial out there for my problem, and if not, is there a program that would at least lead me the way. Thanx for your help.
-HF
I am completely new to the whole repaint thing, and I was wondering if I could begin now. I am sorta aware of how to edit textures, but then again, that is Halo, and that had a pretty straight-forward program, or HaloMapTools. Anyways, I have Photoshop 7 and i used that for the textures. However, I thought that it would be easy enough to just open up the texture files and mess with them using Photoshop, and I am apparently wrong. I get an error message saying that it could not open.
My point is, is there a tutorial out there for my problem, and if not, is there a program that would at least lead me the way. Thanx for your help.
-HF