Believe it or not, but photos are not a legal requiremnt for a PCN served to the vehicle at the roadside, so you would not win on that alone. If you take the matter to the adjudicators, the adjudicator will decide using the civil law test of "on the balance of probabilities", so it's your word against the CEO's. However the CEO has taken photos of your car in the bay, so this shows you were there, and therefore should have read the sign.
By parking in a clearly marked bay, it is encumbent upon you, as a motorist, to look for a bay sign defining the terms of use of the bay. This bay has a single small sign, "Loading Only", which means with not days or times, that it applies 24x7.
