You are using the image as a background. Why not set it as the src property of the button ?
<input src="https://stackoverflow.com/questions/4108983/images/submit-bkg.png" id="searchsubmit" name="searchsubmit" type="image" value="" tabindex="2"/>
When you set the type
as image
the input expects an src
attribute as well..
Reference: http://www.w3.org/TR/html401/interact/forms.html#adef-src and http://www.w3.org/TR/html401/interact/forms.html#h-17.4.1