I've been perplexed trying to understand the following issue. I have several devices and services: specifically, for example, a Macbook, an iPhone, and a Gmail e-mail account. Each provides its own Contacts interface, and there is some degree of syncing among the devices and services.
Somehow, "strange things happen" that I can't explain. For example, people for whom I've never set a contact icon somehow develop a contact icon spontaneously. This icon is sometimes the same in Gmail and on my phone, and sometimes not. (Presumably, the contact in question chose his/her icon, and it was automagically pushed to my device or account.)
Similarly, though I never set an icon for my Gmail account, somehow it just found the icon that used to be my user account icon on a former Macbook that I migrated to the current one, and without ever asking me, set that as my contact icon.
This is especially odd or disconcerting to me for the following reason. Sometimes I get e-mails on which are cc'd other Gmail users who have never emailed me before, yet I can immediately see their contact icons just by hovering over their e-mail addresses in the recipient list. This seems like an invasion of their privacy (and consequently of mine, as I presume the same happens to me). I can't find a setting in Gmail that controls this behavior. Notably, I've never set the "Select a picture that everyone will see when you e-mail them" option; this is unconfigured in my Gmail. The help page implies that if I did set it then I could control who could see it, yet I don't seem to be able to control that without setting it, and though I haven't set it, I still have an icon! How odd.
I'm not especially worried about this, just confused. I'd like to know what the propagation schema is for contact icons. In general, whose icons are being broadcast to whom, and via what services? E.g. do only iMessage recipients see the icon that I choose for a Contacts account in iCloud, and only Gmail recipients see the icon that I choose for a Gmail account? Does syncing contacts across these services push my contact icon between them? What about icons I select for others? What about the icons others have selected for themselves? Etc.
I find it odd that I know nothing about how this works even though I aggressively configure settings for all of my services and devices, and it seems to be a bit of a hole in the universe of privacy settings. Am I missing something obvious?