To make them is hard to say because there are so many ways. Whether you use a 2d program (Photoshop, Corel Paint, Paint Shop Pro...yada yada yada) or you use a 3D program (3DS Max, Lightwave, Cinema 4D, **Bryce**

) and a combination of these and countless others. I can tell you that it's not an easy thing to have your wall make it on this site. It has to be top notch and pretty good quality (not like ThemeXp and their wacky crap) but the community has several people who probably could help you more (Myself for Bryce if you chose to use it

). Anybody else wanna jump in?