He's a lovely looking horse, but on what are you basing a $20,000 price tag?
As far as the photos go, you need much better ones than just you plodding around on him wearing flip flops.
For that price, I'd expect professional shots of him in the show ring. Blue ribbon photos would also help your case, as would them being from a recognized A-rated show and not some open, nonrated one.
Just being Hanoverian doesn't make him a $20,000 horse. He needs to have earned that price tag.