You should be able to disable it. That's a valid input type; it should be supported widely.
<button><img src="[href to image]"/></button>
You might have to restyle the button to stop it being drawn as an actual button... but the button will have a disabled property; and disabling the button will stop the image being clicked/prevent the button from being functional.
Dude, you have the answer above already...the onClick parameter is ALWAYS going to run when clicked... this is NOT form input.... The input tag with type="image" is the CORRECT way to use an image as a button... if you don't do it correctly even when people tell you, then there is little we can do...
Now to do it your way, don't use disable, but hide the button all togther then it isn't being enabled... if they can't see it they can't click it, so your onClick won't be an issue...
OK, but I highly recomend using the proper method of input tag with type="image" parameter set.