Change Folder.current to location of script (.jsx) file?

All,

I'm trying to use some images in my ScriptUI dialog, but I want to use a "relative" path name, and I can't seem to get it to work.

For example, this myScript.jsx works just fine, but as you can see, I'm using an "absolute" path name to transparencyGrid.jpg:

var w = new Window ("dialog");
var f = new File ('~/Desktop/transparencyGrid.jpg');
w.add ("image", undefined, f);
w.show ();

However, I want to be able to use a relative (to myScript.jsx) path to point at transparencyGrid.jpg, like this:

var w = new Window ("dialog");
var f = new File ('../../../transparencyGrid.jpg');
w.add ("image", undefined, f);
w.show ();

But, I can't seem to get this to work because relative paths seem to be relative to the current folder, Folder.current, not the location of the currently executing script.

I say that because when I do this:

alert(Folder.current);

The only thing the alert message shows is a backslash (/), indicating to me that the current folder is the root of my OS, but I thought it would be the folder where myScript.jsx resides.

So, is there a way of setting Folder.current to the location of the actual script?