That's the joy of JPEG.
If you create the images as RAW OR TIFF, edit as TIFF, then save to a highly-compressed JPEG, the JPEG artifacting will be uniform and will help disguise any doctoring deficiencies. The fact that the JPEG artifacts cross over any logical edit boundary just means that it was JPEG compressed AFTER editing, not before.
What I would want to see is a JPEG of the device with proper EXIF tags showing that the JPEG came directly from a digital camera, with no editing. (Or, even better, a RAW image with proper EXIF tags.)
Yes, it's possible to edit EXIF tags, and yes, if someone was good enough to fake this kind of photo, they probably would be able to fake the EXIF tags, too... But it would be better than a random high-compressions JPEG.