Problems Clicking <input type="image"

Could somebody help with me this problem?

I am trying to automate filling in a form and submitting the details. I have filled in the details but the submit button is an image

<p><input type="image" src="images/buttons/medium/btn_Submit_0.gif" class="rollover" alt="submit"/></p>

In my script I am trying this:

_IEFormImageClick($formName, "submit", "alt")

$formName is the form name I have also tried this using the url of the page  but the image is never clicked (or if it is it's not doing the post action of the form)

If I output the code to a message box it is returning -1

I have also tried using src instead of alt but that isn't working for me either.