How does my iphone associate a photo that isn't on my phone with a contact phone number?

Thanks a ton for the responses. But, what is happening isn't on "my phone" per se. People I call say that someone else's photo is displayed on their phone. This happens when I'm using my phone, and obviously my phone number. These people have not added the incorrect photo to my contact on their phone, the phone is "learning" the photo on it's own?

To put it in terms of my phone (although I don't have the problem with the phone displaying incorrect photos for my contacts numbers) is.... I have a ton of contacts that have photos associated with their contact entry. All good. For none of them did I add their photo myself to their contact on my phone.

Note: I'm not on Facebook, so I don't believe that is the link.

It's a bit of a head scratcher for me.

Thanks everyone. Turns out it was Google Apps for Biz account. My ex-colleague is monitoring my old work email (which is hosted by Google Apps). My old work account has been linked to his work account, and has pulled his photo from his personal gmail account. Google going rogue. Ha!

Both people (both Android users) that reported my colleagues photo appearing when I call them have now deleted my old work email account from their contact entry for me. This broke the Google Apps link and allowed them to delete his photo from my contact entry. Wow.

Thanks again for the thoughts.

One has an Android, the other I'm not sure.