TMZ Sports has obtained a photo of Ronald Gasser moments after he shot Joe McKnight ... and the witness who took the photo says he WAS yelling at McKnight's body after the fatal shots were fired.
The person who took the photo says the item on Ronald's blue Infiniti is the gun he used to shoot the ex-NFL running back.
The witness -- who is NOT the same person who spoke with NOLA.com -- says she heard the two men arguing on Thursday afternoon .