I've been staring at chimpanzee and bonobo references for the past several weeks, and as I've been looking for references it's dawned on me how relatively generic and thinly spread the references are on the internet. That's odd, considering it is the Internet. There are a lot of photos of chimps, but very few show them clearly standing up, or directly from the front and back.
I've found a book, "Photographic and Descriptive Musculoskeletal Atlas of Chimpanzees: With Notes on the Attachments, Variations, Innervation, Function and Synonymy and Weight of the Muscles," by Diogo, Patau and Pastor.
It's available on Amazon; sadly at £84 it's a bit out of my price range at the minute, but plenty of useful information is contained in the Look Inside pages!
If you're not as versed in medical jargon, the pictures will do enough explaining on their own so far as an artist is concerned. Be aware that it contains photographs of a medical dissection, so be prepared to look at real tissue rather than illustrations.
On the free side of things, there is a site that contains orthographic scans of just about every ape and monkey skull, both new world and old, for free:
I've compiled a big folder full of different ape and monkey skulls for use in future projects.