I learned that the Octopress built-in Google+ 3rd party plugin http://octopress.org/docs/configuring/#third_party supports establishing authorship and is probably the intended way for Octopress bloggers to add the desired functionality.
To use it you can change the Google+ section around line 80 in the _config.yml
file:
# Google Plus Profile
# Hidden: No visible button, just add author information to search results
googleplus_user: 111234567439674839635
googleplus_hidden: false
replacing the user number with your own, which you can find by going to your Google+ profile and copying the number in the address bar. Then, as long as you have added the URL of your Octopress blog to your "Contributor To" section on your Google+ profile page, your profile image should show up next to search results for your blog articles.
I had erroniously set my googleplus_user url to be the full URL of my Google plus account without realizing it, so authorship was not working. Thus, I sought to manually add the <link>
tag in my original question.