A twitter background is just basically an image. You can use any paint program to make your own. The basic thing you need to remember is that the image is displayed "starting in the top left corner or the screen". Keep this in mind while positioning any design elements like logos and text. What shows up on a visitors screen depends upon their screen resolution. Supposedly most users have a screen resolution that start at 1024×768 So you'll need to experiment. The top area on the twitter profile is also taken up with the twitter menu (about 71 pixels high) The middle twitter tweet box area also is about 760 pixels wide.