With programs like Photoshop, GIMP, Paint.net, Paintshop Pro....things like that. I personally use GIMP. Photoshop is expensive if you buy it. But if your willing to torrent, then so be it. EDIT: Also the things at the bottom of people's post are called signatures. The one that goes under your name to the left is your avatar.
Tinkering I find is the best way to learn. Or check youtube.com for tutorials. That helped me as well.
Most of the time, youtube is not a good source for tuts. http://www.good-tutorials.com/tutorials/photoshop Try that, it has a cool bunch of tutorials for PS.