1- prepare an image you want to put as background
and name it for example : 1.gif
2- upload it to images folder via ftp
3- go to admincp - styles and templates - choose the skin you want
and go to the css
4- from css -- go to WYSIWYG
put the 1st box : background : رمز PHP:
#ffffff url(images/1.gif) no-repeat
الصور المصغرة المرفقة 
FavpD : put a background for your reply