You just need a good graphic editing program. Windows Paint won't cut it.

There's Photoshop, of course, which is what I use, but few people have it because it is so expensive. However, there is a free Photoshop clone, called Gimp, at gimp.org

There are other free programs that are good, too. I heard some use the one from getpaint.net although I haven't tried it and I don't know if it supports the .png format.

Whatever program you use, you need to take the time to learn how to use it. To make mods you need to know how to use the basic tools at least, and it helps to know how to work with layers and opacity.