By default, the subscription form has a simple button that says "Subscribe".
However, if you want to use your own graphic subscribe button, here's how you can do that.
Find this line in your form's code:
<input name="FormButton_Subscribe" value="Subscribe" id="FormButton_Subscribe" type="submit">
Replace that line with this code:
<input type="image" src="http://www.yoursite.com/images/yourgraphic.jpg" alt="submit!" value="submit" />
Make sure you change the path http://www.yoursite.com/images/yourgraphic.jpg to your image's full URL and you're done!