When the resolution of the source material differs or lossy compression was involved, it can be possible to differentiate between the parts. In it's easiest for

When the resolution of the source material differs or lossy compression was involved, it can be possible to differentiate between the parts. In it's easiest form you subject suspected forgeries to lossy compression and check with a visual diff whether the loss was uniform. Search for "jpeg artifacts forgery" to get an impression. In this case there is no question.