background picture in default user interface

how do i make my own default layout with my picture as the background

Most of the layout of default user interface is defined in the skins in the includes folder. You are free to change them all, so the answer to this question depends on how you design your user interface yourself. But assuming you follow broadly the same subdivision into skins as the standard layout, you will probably either set a background picture in style/phppeanuts.css, or set it in skinHeader.php and maybe in skinHeaderClean.php. On how to set background pictures in general see http://www.w3schools.com/html/html_backgrounds.asp and http://www.w3schools.com/css/css_background.asp.