Before we get into appeals, I note this is a British Gas van - who is the registered keeper of the vehicle, British Gas? I ask, as this may affect the approach to appeals.
can I challenge the fast that the pictyure only shows my van parked up without the sign and therefore there is no contravention
If you do, they'll probably refer you to one of the other pictures taken at the time, showing the vehicle parked directly next to the sign:

A far stronger challenge would seem to be that the ticket was issued for "parked in a Contractor / Visitor Bay without a valid permit", but the terms on the signage do not require a permit to be displayed. Do you have (or can you acquire) evidence to demonstrate that the driver was working as a contractor at that site?
Here's a better image of the signage in case we need it for the future;
