How to access textures?

Hello, i’m new to these forums. But NOT new to Garry’s mod.
I’ve modded several games since like 2004, but the only thing i really wanna mod, is Garrys Mod.
I have a couple of texture ideas for people like Heavy, or G-man.
But i do not know how to, or neither do i know what the texture file extension is…
So i hope someone here can guide me to a tool, and tell me what file extension the textures are.
Thank you!