I would suggest instead of using
<input type='submit'>
use
<button type='submit'>
Button was introduced specifically bearing CSS styling in mind. You can now add the gradient background image to it or style it using CSS3 gradients.
Read more on HTML5 forms structure here
http://www.w3.org/TR/2011/WD-html5-20110525/forms.html
Cheers! .Pav