that horse isn't long and low, he's all strung out. TRUE long and low makes a horse pick up speed the lower his head is.
WP, however, is a bit different... I'm not a huge fan of that picture at all, really.
Anywho. If you have to keep your horse at a slow jog to sit the trot, then you are not balanced. I'm sure you feel alright, but until you can sit a trot (smoothly), don't expect too much from sonny. It's a hard skill to master; make sure you aren't using your hands to keep your balance at the trot, too.
I don't want to sound mean, but there's really no other reason why he would stick his head up other then you cannot sit the gait (it may LOOK fine, but it doesn't feel fine to him). If it was training, then it would get progressively worse with speed (like the canter)--just practice makes perfect.