Save it to your PC, then right click and select properties. You'll get a whole load of info, and should be able to tell whether it was taken with your phone, or downloaded.
If it was taken with your phone one of the bits of info will be the latitude and longitude, which will tell you where it was taken. That should narrow down the possibilities as to who took it.
If it was downloaded, the properties should tell you which site it came from, and when.
I don't think it's possible to hotlink on the iPhone, but just for the record, hot linking is when you save the URL of the photograph rather than the image itself. This means that if the original pic is changed, your copy also changes. I'm sure this only applies on computers, and that the image is always saved on the phone.
Some dumb politician (I forget who) Googled for a picture of himself, hot linked it to his website and looked pretty stupid when the owner of the photo changed it for something rather less appropriate.
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