Face2Face for Mac can look up a specific image on Google+, Flickr, and Facebook and link it to the corresponding contact in your Mail.app. It installs as a plugin for the default Mac mail app, with which it integrates seamlessly. Overall, it's fast and easy to use.
While the Face2Face installation process lacks information about how to configure the app, a guided configuration shows you the main features and setup once you open Mail. The app has the option to show the contact icon on every listed e-mail and lets you choose what social networks the images are sourced from. The image linking is very quick and gives you the option to include company images for e-mails from corporate accounts that are not linked to any social network. Another handy feature is the "Clear Image Cache" button, which begins a new search, letting you easily update profile pictures after they had changed.
If you use Mail as your main e-mail application manager and like to have your contacts' information as complete as possible, Face2Face adds a new level of dynamic customization. It's definitely a useful add-on for your Mail application. Something really nice to see would be the ability to export contact images to the Address Book.
From Feingeist Software:
Apple's Mail normally uses your personal address book as its source for images, which are shown for emails from the corresponding people. Face2Face is a plugin for Mail, which adds new sources for people's faces based on their Google+ account, their Flickr account, their Gravatar profile or based on Face2Face's own database.