My form isn't returning any results when submitting the form if the user hits the <spacebar> or the <enter> key on the keyboard when tabbed over the image submit button.

Code:
<input name="Submit_notes" type="image" value="Submit_notes" src="images/pp_but_blu_addentry.gif" />
Should I make it so the user can't submit with the keyboard? (if so how) or is there another way?