Javascript color picker yields strange errors

First off, I am by no means Javascript-literate... but this little tool
looks promising or what I have in mind:

I have it working ot the point where my users can select their
preferences, click a field to change color, and the javascript control
appears, except that I don't get the slider control shown, nor the
shading on the color, etc, etc.

In my logs, I see

ActionController::RoutingError (No route matches
"/userprefs/edit_me/images/hue.png" with {:method=>:get}):

where the colorPicker.js is trying to build the control with statements
like this:

blah blah blah...
'<img id="colorpicker-bg" src="' + this.options.IMAGE_BASE +
'pickerbg.png" alt="">'

blah blah blah.

I've tried setting the IMAGE_BASE in the _form code where the javascript
controls are created like this:

    new Control.ColorPicker("userpref_site_back", { IMAGE_PATH :

but the path ends up including the controller and action.

I must be missing something. Of course, if I knew what it was, I
wouldn't be missing it...

Thanks in advance for any ideas.