For jumping your position looks great. In picture #3 (horse on flat, rider red shirt) horse is hollow. To fix that push him forward into reins, soften elbows about 1/4 inch forward and ask him to bring nose in. If he doesn't then using reins flex him (quickly) inside/outside then a tad more leg to push him forward.
What you want is to have him bring his nose more onto the vertical and step underneath himself with his hind legs - will balance him more and enable him to use his back more effectively. If he's really working well rider will see a "bulge" in the middle of both sides of horses neck. When my mare hollowed constantly I had to take her to a long and low position to re-establish proper contact (i.e. Get rid of the hollowing evasion) then slowly bring her poll back up to the proper height.
Maintaining a proper use of the back also makes horse more powerful for jumping and builds up his topline.
Nice jumping pictures. Advanced riders (George Morris) do not use the crest release - that's more for beginners who need the horses neck to lean upon. I like that you're using that release as it display the fact that you have more core strength and do not rely upon the horses neck for balance.