Your #masthead-images div has a different height on different pages because there isn’t a set height on it. The pictures on the Dolphins pages are smaller than on the Manta Rays page so the #masthead-images div is shorter causing the #main-content div to display higher up.
Give it a set height and adjust the margin-top on the main content div to line everything up.
Or you can give the header a set height and give it overflow:hidden. Then adjust the height so everything is visible, then adjust the margin-top on the main content div to line everything up.
Either way works