When either you or your camera selects an f-value (e.g. f8) for any given scene it determines what will be in focus in the area 'before' the subject and how far the focus will extend 'behind' the subject. In this case the camera chose to focus on the neck of the lizard. If the f-value had been higher f16 for instance, the sky would have been darker and more of the lizard would have been in perfect focus. You or your camera chose f4.5, which limits the 'depth of focus' to only a small area. a higher (smaller) aperture will create a greater depth of focus and get more of your subject in focus. Lizards are fast, and difficult to sneak up on and shoot. I recommend shooting on a cameras 'manual' setting and studying the "sunny 16 rule" as well as the "zone system" You don't have to be a perfectionist about it, but having a little information can make the difference between an 'almost' shot and a personal best.