how to disable button onclick to get rid of double submissions?

I need to figure out how to disable the submit button once it's click so that people can't submit twice. I have this:

<input src="images/n2.jpg" name="submit" id="submit" type="image" onclick="this.disabled=true">


Doesn't seem to work though.

 

its either:

this.style.disabled = true;

or

this.style.enabled = false;

i always get those two confused..

 

<input src="images/n2.jpg" name="submit" id="submit" type="image" onclick="this.disabled='disabled'">

HTML:

this should work