I can walk you through a quick tutorial if you'd like, just teaching you all of the basics. Or you could look stuff up on YouTube, ect. No, you don't need the image first, and usually it's best if you set a size, then ADD the image AFTER you've started.
Well, I use Gimp 2.6, which is a free thing you can download from the Internet, along with fonts, brushes, ect. If you're willing to dish out some money, you can get the REALLY good stuff, like Photoshop, which I want. ;_; But there are plenty of people who make gorgeous graphics on Gimp, too, like c a l i l i l y.