It is sadly impossible to to prove when an image (or any file for that matter) originated. It is possible (if the author wants to) to prove that a file existed prior to a given time by signing the file from a third party time stamping server (through which the third party proves that the file existed at the time of the signing) but such information is not automatically possible and can easily be stripped.
I am also an IT Security guy and there is no possible secure way to prove the creation date of any file if the user controls the system creating the file with current technology that I am aware of. The best bet would be a device with a locked clock that would have a hidden key store that the user shouldn't have access to and create a signature based on this so that they couldn't fake their own signature, but since the key must still reside in the device, it is still feasibly possible for someone to break as all the necessary information is in their possession, even if it is hard to get to.
As far as detecting an amateur job, there is generally a file creation date in meta data of the file system itself that could be examined and compared to the EXIF metadata, but if they are good at it, they will have altered both and there is some possibility of the file system values getting lost depending on how the file is transferred, so it may not even be reliable.