The issue is that the bottom of people's faces are cut off in the Pedigree view, as you can see in the screenshot below.
Now, skeptics may opine that this must be a problem with my photos. Not so :) The bug appears to be unique to the Pedigree view: the pop-up that appears when you hover over a person in Pedigree view, as well as the Person Profile page -- which all use the same photo -- all display as they should as you can see in the screenshots below.
So anyway, the good news is that I received an e-mail reply a couple of days ago from Jen Hodnett, the Ancestry.com product manager for the Tree Viewer, who stated
"you’ll be happy to know that we’re in the process of fixing this. Look for the update in the next week or so."
Yeah! Soon, we won't have our ancestors peering over the fence at us!