It's also impossible to edit a picture on a digital camera, FWIW...

That depends somewhat on what kind of editing is meant, and on what kind of camera you're using. With the advent of numerous printers that allow printing "direct from the cameral" many digital cameras have added features that allow rotation of views, cropping, addition of notations and smart-assed pasties for decoration, color shifts, and other manipulations. The "cute" features probably are more likely to be found in newer phonecams than in more standard digital cameras.

It is also quite true that an image can be downloaded for editing and then reloaded into the camera.

The EXIF data imprinted in the image by cameras relies on the date/time setting for the internal camera clock, and maintaining correct settings is often ignored by users. There is some additional likelihood that a digital phone-cam will have an accurate date/time since the time is critical to phone communication and is usually synchronized with the service automatically. "Service time," however can be significantly "off" from real UCT time for some services, since an absolute reference isn't required - only agreement with other phones on the same service.

The EXIF data also can be edited, although most simple programs will add an "edit date." More sophisticated programs reportedly can "make like new" EXIF data, although I have no experience with one that does it.

Although some phone cams are very good, typical ones are not high resolution, and a photo of a photo, done carefully, can look like an original, and will have the effect of creating an "original" with a phony EXIF dataset. The internal clock usually can be reset manually to get any time you want, as long as the photo is taken before the synch resets it. Most synch systems make adjustments in increments with a maximum change per adjustment, so it may take at least several hours for a manual setting to be completely wiped out.

I do agree with Nick - Photoshopping an original/altered picture that looks like a photograph is very difficult without obvious editing artifacts, and even for an expert is extremely time consuming, so the odds in this case are that any editing was done via a new collision or with a sledge hammer.

John