..Hope this is the right Forum room for this post..

Alright, Well. I have been using Sphax for a month. And quite recently i started using mods. And it was actually a surprise for me how Sphax is almost only great Texture Pack that supports mods.
Anyways, I'l get to the point , i would LOVE to start texturing mods as well. But i honestly barely any idea how.
So far i know so much: I get the .png file i wanna re-texture. And i simply use paint/gimp.
But now if i want to make that file bigger? Want to make that x64.
So can you give me the basics, and anwser following:
Do i need any special software? If that software costs, are there free ones i can use for re texturing.
What are the basic rules of making a texture? Do i need to ask the mod maker, before i texture the mod?