In all wp.com themes, the header image or the blog title works as a link back to your homepage.
Some themes allow you to hide the actual blog title and tagline (see list in my post Header images). That’s useful if you wish to add your own header image with the blog title incorporated in it as a graphic.
But if you do that in Vertigo, you’re left with no link back home on or near the header: the home link is the title, not the image, and the theme has no top nav menu or sidebar either (in which a Home tab would or could be included).
You can turn your whole header image into a home link by adding a Text widget to one of the footer widget areas and pasting the following code into it:
This will produce an invisible linking rectangle in front of your header image. (The middle line of this code says create a rectangle in the dimensions I’m giving you and turn it into a link to the URL I’m giving you; the outer lines say take this thing and place it where I’m telling you instead of where the code is.)