I think though the difficulty this thread is illustrating is that there is no agreed / accepted welfare standard on rider weight - without that how can photos prove anything, it's all going to be subjective opinion on what's 'fine' or not. I think for most of us we accept the 15% guide as the best we have but acknowledging it's not based on particularly robust research - there are a lot of compounding factors that it's hard to eliminate/ control for such as saddle fit, rider skill, fitness and soundness of the horse. It all also gets a bit existential doubt-y, how do we define 'harm' or damage to a horse's welfare from being ridden, there's an argument that says no horse is evolutionarily designed to be ridden, so no amount of riding is 'good' for them - similar sorts of arguments apply to things like children's contact sports or competitive /intensive gymnastics or dance - arguably the safest thing is to just not do the sport at all but that isn't helpful in designing safety standards!
I still personally believe that the 15% is a good rule, and future research is only likely to suggest a downwards amendment not upwards. I.e. even if all possible factors are positive (well fitting saddle, skilled/fit/balanced rider, horse fit and in good health, light/short work, suitable surface), the 15% guidance would still be your maximum/limit and if some of those factors are more towards the negative e.g. beginner rider, galloping/ jumping then it should be less weight for the horse to carry.
With that in mind I can't imagine how any photo is going to prove to me it's fine for a 12 stone rider to ride a 12.2 pony, it's just not physically possible for a (healthy) 12.2 to weigh 80 stone/ 500kg and that's before you account for clothing and tack and any excess bodyweight the pony is carrying. And I would never, ever encourage people to post identifiable pics of their children on public forums, and frankly would find it quite creepy for us (and remember you don't know who any of us are/our motivation) to pick over details of her body in that way.