Given that a ticket was absolutely displayed and easily visible from outside the car there are two possibilities.
1 the photos were digitally altered to hide the presence of the paid ticket receipt or
2 more likely that the photos were angled to hide the ticket.
3 a rather dozy attendant didn't spot it (I'm not saying this did happen, but it's another possibility that you would need to rule out if you were going after them for the deliberate false representation point)
I think you're correct that #2 is far more likely than #1. I'd be amazed if Alliance considered it worth the effort and legal risk to doctor a photo for the sake of £100, they make enough money without resorting to that. An attendant who is potentially paid a commission for each PCN issued on the other hand...
More generally, I'm not sure it matters which of the potential possibilities is correct for the purposes of your defence and witness statement (and accordingly expending too many words speculating in your defence/WS might be counterproductive) - your job as defendant is to swing the balance of probabilities such that the district judge believes that on the balance of probabilities a ticket
was displayed. It is probably easier to demonstrate
how a ticket might have been clearly displayed yet not be visible in the photos, than to demonstrate
why (e.g. trying to demonstrate intentional deceit on the part of the attendant).
A very minor clarification - no magistrate will be involved, this is a civil case.