They've previously published that first-messaging rates are drastically lower for profiles without pictures. IIRC even pretty bad pictures are better than no pictures at all. That would be a huge confounder.

There is something on that in that blog. It's a little more complicated. They turned off pictures, and studied the rate of message responses, and the satisfaction users had with the resulting dates. They concluded that users strongly preferred to message good-looking users, but that preference for looks was not borne out in their satisfaction with dates.

It looks as if it might be worth to manually disable pictures (e.g. with HTTP switchboard) and browse profiles only seeing text.