The article title gives the puppy image credit but I think the additional text had as much if not more of an impact on the conversion lift than the image.

  "Start your 30 days free trial" text 

vs

  "*Sign up now* 30 days free rial.  Pick a plan & sign up in 60 seconds.  Upgrade, downgrade, cancel at any time." with a puppy image.

The second version is so much better because it tells the viewer what to expect... the 60 second signup, flexibility in changing plans, no long-term commitments. The dog image does help since he is looking at the call-to-action, which also draws the viewer's eyes there too.

It is a great post and just goes to show how testing can really help boost conversion. It's often a lot cheaper to grow your business by improving conversion rates than by increasing traffic.