Helping people with computers... one answer at a time.
Digital photos often include information which may help establish their origin, but it's information that's also easily altered or removed.
I have a friend who is a photographer. A few years ago, a publisher decided to publish a book using his image without his permission. When he saw the book on Amazon up for pre-order, he contacted them and they modified the book, claiming that they never printed any with his image. But, his image shows up all over the web as the cover of the book. We've called around to the bookstores that are selling the book and it always turns out the image is wrong and the book they have is the one without his picture.
My question is: how much info is stored in those images? Could we find out where they came from and who made them?
There's definitely a lot of information that could be in that image. The question, though, isn't really how much there is, but rather how much survived and whether you have access to it.
I'll look at how information might be included in images and why it might get lost or altered.
I'll also touch on what I think is the only practical solution for a case like this.
If you can get the original .jpg file of the unmodified image, there's potentially quite a lot of information included in the file. Along with the image itself, there is EXIF information. In this, there might be things like the camera make and model, the settings used, the date and time of the photo and - in some newer cameras - even the GPS coordinates of where the camera was at the time the photo was taken.
To say that this information is "embedded in the image" is actually incorrect. More accurately, it's information that accompanies the image in the image file. There are methods for embedding or hiding information in the data that makes up the visible image using something called steganography. But that's a form of information concealment that doesn't apply in day-to-day photography.
The EXIF information that most cameras include is clearly placed there as part of certain file formats, such as .jpg; there's no attempt to hide it.
Unfortunately, while EXIF information is carried with the image, it's not part of the image. If all you can do is see the image without examining the actual file, you won't have access to this information.
Another problem is that the image you're seeing on the web is very likely to have been heavily processed. Without seeing it, I'm guessing that it was resized and probably had some text or other graphics laid on top of it. From a digital perspective, that's pretty much a different picture; the resulting data that represents it will be completely different.
The tools that perform those operations might alter the information. That means that while your original photo might say "Taken with a Nikon D300", the processed image might say "Taken with PhotoShop". Other information, like the date or location, might be similarly altered.
Or the information could be stripped completely. EXIF information is not required and it's possible that some operations might remove it completely in order to make the image file smaller for use on the internet.
Nonetheless, you could get lucky. Download one of the image files you find on the web and use an EXIF viewing tool to see what it contains.
If you're using Firefox, another approach is to use the EXIF Viewer plugin. After installing the plugin, right-click on the image that you're viewing and select "View Image EXIF Data". Then scroll around in the resulting dialog box. Perhaps there's residual information in it that would confirm the theft.
I'm no lawyer; let me be clear on that. I can't advise on legal matters, and you should seek actual legal counsel if you feel that this is worth your pursuing.
If I were in your shoes, I know that digital proof would be very nice. But, in the scenario that you outline, I'm of the opinion that it's unlikely. There are too many ways that the information that might aid your position could be altered or removed simply as a side effect of re-using the photo as you describe.
In a court, however, you might be able to pull in an expert witness to say "Yes, that picture is this one".
That, or the threat, might be what it takes.
Or it might not be worth the expense and effort.