The main images are called renders and are often from official wallpapers. The renders are cut from the wallpapers and are put into a new image with a transparent background. These images can then be imported into a smaller canvas along with other images or with user-generated brushwork. There are many different styles of creating sigs, but the generic ones are more abstract, like Magus's current one, or overall compositions like mine which depict a scene.

Its all about your own style and what you feel comfortable with. Grab a copy of JASC Paintshop Pro or Adobe Photoshop and design what feels natural to you. I think my style is quite unique, but I started by following tutorials, because following them can set you in the right direction and will enable you to pick what styles suit you best. Magus has some good tutorials and there are plenty of sites out there that have step-by-step instructions for beginners.