You need to adjust the <a> tags around the image to do that, though it's probably not a good idea to send customers to PayPal from your homepage before they've had a chance to buy anything, perhaps a normal image may be better. But this is how you do it anyway.
HTML Code:
<a HREF="http://www.paypal.co.uk" target="_blank"><img src="http://www.paypal.com/en_GB/i/bnr/horizontal_solution_PP.gif" border = "0" alt="PUT SOME NICE JUICY KEYWORDS HERE"></a>