Speaking as a ground pleb myself, the issue is more about using personal electronic devices within the aircraft footprint (which is forbidden) rather than the actual photo itself.
https://publicapps.caa.co.uk/docs/33/CAP748.PDF
2 Portable Electronic Devices (PEDs)
2.1 There are three primary risks associated with the use of PEDs in the vicinity of aircraft:
Fire, Distraction, and Aircraft System Interference.
2.1.1 Fire: The risk of a PED creating or inducing a spark of sufficient intensity to ignite fuel
vapour released during fuelling is extremely remote under normal circumstances.
However, licensees should be aware of the proliferation of below-specification
mobile telephone batteries that have the potential to fail dangerously. It is not known
whether such a failure would be of sufficient magnitude to ignite a fuel/ air mixture,
but licensees should be aware that such a possibility exists. It is recommended that
they consider the circumstances under which such an event might occur on the
apron, and mitigate the associated risks accordingly.
2.1.2 Distraction: The known potential for a PED user to be distracted presents three
associated risks:
a) physical contact with the aircraft by the distracted PED user could cause damage
or injury;
b) equipment being operated by a distracted PED user could cause damage to an
aircraft; and
c) PED users, distracted while performing essential safety related tasks, could leave
those tasks incomplete or unattended.
NOTE: Licensees should be aware that the hazards at 2.1.2b) and 2.1.2c) above are
associated with actions or inactions by apron staff, and carry the potential for the
effect to be concealed until a stage of flight where the safety of the aircraft could be
compromised.
2.1.3 Aircraft System Interference: Reports have been received that the use of PEDs
close to modern aircraft can interfere with fuel gauges, some navigation equipment, and can cause spurious fire warnings in cargo/baggage holds. Such interference could
contribute to the risks associated with any of the following:
a) an overweight take-off due to excessive fuel;
b) a flight with insufficient fuel;
c) navigational errors; and
d) a degradation of confidence in the aircraft fire warning system.
2.2 Licensees should prohibit the use of PEDs on the apron area, or should restrict their
use to clearly defined and promulgated circumstances that mitigate the risks
associated with their use. These mitigations should be considered against the
volatility of the fuel type involved, the proximity of vehicle and aircraft vents, the
circumstances under which they may be operated, the category of the hazard, and the
provision of an alternative non-interfering communication system. Passengers
boarding or disembarking the aircraft should be discouraged from using PEDs when
outside, but in the vicinity of, the aircraft
If I refuse to delete an image taken, what happens next?
Though unlikely, you could find yourself offloaded from the flight if you fail to comply with reasonable safety requests. 9 times out of 10 the captain would dispatcher on the decision to offload. I've done it many times myself for smokers.