Generally, if a picture is updated the original contributor is still credited for the submission and the new submitter (whether that be a CG admin or a user upload) is shown in the "updated by" under contributor.

If in fact we replace a submission because a newer image of the same item is superior in quality, whether it be condition or the quality of the image, or that the original submission was incomplete, then the original submission will be replaced giving credit to the latest submitter.

Hope that makes sense,
Eric