I believe that the saddle fit my long-backed horse well, it's solid on her and fits just as good as my western saddle does, even though she doesn't have much for withers... The saddle sits off of her wither and for what I can see, she doesn't fuss about any issaues with the fit.
It looks like you are placing it too far forward and it also looks too narrow as it is tipping backwards (due to it being too high at the front). It is easy to measure a saddle, just measure from the stud to the centre of the cantle. It doesn't look right for both you and your horse in those pics. Great advice from Barry Godden - Ideal saddles are my favourites.