I'm no expert at how saddles fit but I'd say that it looks like it fits pretty well. Try putting it on her and riding at all gaits, then when you get off check to see if it's slipped back at all. If it has that probably means it doesn't fit as well as it could...
In my opinion, the saddle appears a little bit too wide. However, this isn't necessarily a problem. You can add extra padding to make a wide saddle fit. A narrow saddle, on the other hand, will never fit a wider horse.
My TB is also too narrow for my saddle. I bought a built up pad and it fits perfect. I can still use my wide saddle on the wider arabs and appies with a regular pad.
In the pics with the pad, your saddle looks fine. When you are riding in it, check the gullet and make sure there is enough clearance between the saddle and the withers.
It's a little difficult to accurately tell with your horse eating grass but judging by the pics you supplied, I think it fits fine. Aside from the gullet seating above his withers, I like to follow the line that the leathers make down the flap to the girth.
That line should be your leg and how it lays will determine where your heels and knees will be placed. Whoever owned the saddle before had it fitting their horse properly. Is the conformation of your horse the same as the seller's?