For the 'now playing' background you can open up your nowplaying.dis file in notepad and change the background url(filename);
The filename can be any jpg, gif, animated gif or png in your skinfolder.

body {
background-attachment: fixed;
background-position: left top;
background-repeat: repeat-x;
background-color: #000000;
background-image: url(nowplay4.jpg);
}


or if you open the playlist.dis you can change the playlist background and up next area background images:


body {
background-attachment: fixed;
background-position: left top;
background-repeat: repeat-x;
background-color: #000000;
background-image: url(nowplay4.jpg);
}

........

/* AutoPlay UpNext Display Definitions */


.upnext {

background-attachment: fixed;
background-position: left top;
background-repeat: repeat-x;
background-color: #000000;
background-image: url(nowplay4.jpg);
}