Using form elements of type "Image" in JavaScript with Navigator 4+

I'm building a form that includes an element like: <INPUT TYPE="Image"...> and I need that element to capture mouse events under Netscape Navigator 4.

I can't find a way to address that object:
document.forms[0].elements.length gives "0",
(there's only one form, and that element
is the only one).
document.images.length is also "0", so no
joy there. In the Netscape documentation,
"image" isn't even listed as one of the form element types.
The form works! The image shows up, and appropriate things happen when I click on it! I just can't find a way to manipulate the image element with JavaScript URL

Any clues or workarounds? With IE4, it's easy!

Thanks very much,

Robert van Waasbergen
robertvw@aeds.com