Now looking at your image (<input type="image"...) it is clear that this image is not designed to actually perform any function by itself. It is likely that a javascript function elsewhere has tied itself to this image to make it clickable.

No, <input type="image"> behaves the same as <input type="submit">, just instead of being a plain button w/text, it's whatever image you specify. So it does perform a function (submit form) by itself -- no javascript is involved. Note it also submits the coordinates of the image that were clicked, too.

The HTML Help page is here: INPUT - Form Input (search for "graphical submit" to get to the right place in the page)