How do I limit the active field for the image link?

Since the icon for the "Powered by Drupal" block is not centered on the page, i thought i would create my own block to center the icon.

I then made the icon a link. However the active field for the link extended for the whole width of the page. And i can't understand why. My browser is Firefox 3.0.1.

Since the active field for the "Powered by Drupal" icon is limited to the icon, i thought there might be some special code to prevent the problem i'm encountering. But there isn't.

Thanks for any clues,
Tom Sherlock