Error when changing from button to image button...

Dear friends :

I have a form and a javascript to control the inputs after clicking the "submit" button :

This javascript verifies that some conditions must accomplish when one of the checkboxes is checked, for example. It works perfect when I use the typical submit button: it verifies the checkbox, and tells you to fill a text field when you forgot it... however, as soon as I change the tag of the button in order to customize it with an image, it fails : it does display the javascript verification, does remind you to fill the text field, BUT then it reloads the page and unchecks all the values that already are filled by user...   any explanation ?

This is my typical button:

<input name="button" type="button" onclick="Validar(this.form)" value="Continue" />

This is my image button:

<input type="image" src="img/btn_continue.png" onclick="Validar(this.form)"  width="101" height="22"/>

It is the same !!  why does it fail ?? thanks in advance for your answers...