> AFAIK the picture is only shown if Google is reasonably confident that it is actually you trying to log in.

I didn't know about this feature, but I often saw pictures of people I know when I tried. Apparently, it seems that for example just sharing IP address is sufficient to trigger this reasonable confidence. Not sure what other ways there may be. But indeed, it didn't work for a few random strangers from LKML I just tried.