I know from my experience making a cat skeleton that you will usually not find very good reference pictures of animal's skeletons and individual bones, you may have to go on major medical college or veterinary websites to find anything close. Good luck on the pelvis, looks like you may need all 6 views to get it accurate. Something you can do to make the reference images eisier to find is to take ones that are close to what you need and edit them to make them more easily visible by adjusting sharpness and contrast, changing background color, and fixing perspective errors. You can do stuff like that in GIMP.
If you know someone that is a cowboy or horse fanatic you can ask them if they have a skull that you can borrow.