A couple of ways to stop client side validation firing, especially when using non asp.net controls:
Set the onclick event of the image/button etc to be onclick=”return false;” then instead of using the onclick event to fire a method, use the onmouseup event instead i.e. onmouseup =”javascript:do originalmethod(this);”
This will stop the page posting back but will fire the javascript method for the control in question. This comes in useful if you have a contact form with validation and a separate control i.e. for directions where you can enter a postcode and use javascript to fire off google maps in another page.