You're going to need an image editing program of some sort. Adobe Photoshop (http://adobe.com/
) is definitely the way to go, but it's quite pricey. If you can't afford that, then Paintshop Pro seems to be the popular fallback, but the Gimp will work as well.
Unfortunately, you're either need to build your own logo (I don't think that the logo file is in the subBlue SDK, someone correct me if I'm wrong), have a friend create it for you, or pay someone to create it for you. If you're looking for something similar, I suggest using subBlue (http://www.subblue.com
), as they designed the logo and theme for all of phpBB.