[Javascript] text input field acting up

[Innerlab]

This is more of a CSS question, but I am not in any mailing list for that. In any case, the style changes I am dealing with now are
quite dynamic and involve javascript.

Below is an example of image parameters that can be changed using javascript.

var jsPath = "img/myimage.jpg"
var jsHeight = 20
var jsWidth = 200
document.write('<img src="'+jsPath +'" width="'+jsHeight +'" height="+jsWidth +" />');

Now, how can I change such parameters when the image it's used as a background ?

Example:
div.mydiv {background-image:url(img/bkg_01.jpg);}

Any ideas?

Thanks in advance.