First you need to upload the background you want to use to a server.

Here is my free image host. It gives you a direct link code to your image after you upload it.
www.candiecoded.com/ImageHost/

After you upload your picture, right click on it and choose properties.  Then you can copy the address of the picture.... ( or copy the direct link )

When you log on to your MySpace or IMVU account, click on Edit Profile.


In the About Me area Paste in this code

<style type="text/css">


body{background-image:url("http://www.yourpage/yourimage.gif");
background-attachment:fixed;}

</style>

<style type=text/css>


Edit the code that is shown in purple here to your image that you uploaded.