This is a bit of a tutorial/idea, but it requires sacrifice of the field:website. This idea includes profile background change at will, and all it requires is knowledge of css.
example/mock-up:
Code
$BODY$ <---just a quick replacement for all the codes that go in a profile page.
<div class="userbackground"></div>
The css:
Code
div.userbackground {background:url(<?if($_WWW$)?>$_WWW<?endif?>) repeat-x repeat-y;z-index:20;width:100%;height:100%;}
$BODY$ {z-index:21;}
If you are an avid html/css validator, you might want to put the css code into an inline stylesheet so the code parses without having errors.
Basically, you're using a z-index property to go behind your design and/or repeat the user's desired $_WWW$ or background image depending on how large their bg is.
Alternatively, you can do this:
Code
<img src="$_WWW$" style="z-index:20;width:100%;height:100%;"/>
<$BODY$ style="z-index:21;"></$BODY$>
The only problem is that if the image the user has submitted in placement of the Website field is smaller than the browser width, which in that case ANY, if not all images are like that, will be stretched. You can however, do the min-width/min-height solution for it or some form of javascript to resize properly and repeat if necessary. Again, the $BODY$ is just my laziness to copy the profile page html layout.
I'm going to experiment with trying out the other fields instead of website, seeing as the website field is important for communication of users. ICQ seems the ideal replacement, but it only allows numbers.