The first method for determining if an image has not been tampered with is to look at the metadata of the image. Metadata, a term often used in the SEO world, is data embedded with the image itself. For example, any picture you take with an iPhone has metadata that shows the exact time and GPS location of when the photo was taken (I discovered that while watching an episode of Dateline). With images this metadata is referred to as EXIF data and contains a wealth of info on the photo itself including the type of camera that took the photo and the last program used to handle the image. This means that you can see if Photoshop has been used on the image but the only drawback is just looking at the EXIF data while alter the pre-existing EXIF data, not that big a deal unless you don't want people to know you were looking at this. I have no idea when this would be the case but many websites listed this as a drawback? To view the EXIF data you can download a Firefox EXIF Viewer Plug-in that allows you to quickly view this data while browsing the web. You can also enter the URL of an image or upload the image file from your computer at Jeffrey's EXIF Viewer.
Another way of determining the authentication of an image is to perform an Error Level Analysis (ELA) of the image. This identifies the different compression levels of an image. Every time an image is saved another compression level is added but if the photo itself has not changed these compression levels should pretty much look the same. Confused?
While I was, so here is a clear cut example, using the online ELA Tool called FotoForensics, I am able to enter the URL of an image or upload an image from my computer and view the ELA. The ELA of an image that has not been modified will look pretty much the same, as you can see with the example below:
Yes, that is a picture of Biggie in one ugly sweater, and here is the original. As you can see the images pretty much look the same:
Now let's look at an image that has been altered and what the ELA (Error Level Analysis) looks like of an image that clearly has been Photoshopped. In case you are not that alert this morning a picture of Batman has been added to the image and you can clearly see this in the ELA of the image:
Another method for determining the authentication of an image is to look at the estimate JPEG quality level. Each time an image is save the JPEG quality level lowers so while this not a fool proof method it still can give you a good idea. FotoForensics also has a JPEG Quality Level analysis in their tool.
And the final method is to again use another FotoForensics' tool that somehow determines what the original image file looked like. I could not read anything explaining how this was done but looking at my previous Biggie in an Ugly Sweater example, there is definitely a difference between the two images, can you guess which is the original?
Yes it is the second one.
And in case you were wondering the image was real.