Automattic/site-logo — pull request #7 by BinaryMoon (2014-07-30 13:58:01+00:00)
Title: Update template-tags.php

I think giving the anchor the class will be much more flexible than using the image. You can still target the image specifically using .site-logo img, and you can target the wrapper.

The other option (to save breaking existing themes) would be to add a new class to the anchor (perhaps .site-logo-wrapper).