As with living things, the undead have the same basic anatomy. In general of course, there will always be the strange creatures who don’t always follow the rules; in fact the undead is the best subject to bend the rules of proportion and anatomy but if you want your human zombies to be convincing, knowing basic human anatomy is your best place to start.
There are many sources to get started, I personally like medical books and journals like Grey’s Anatomy, of which there are several versions available with illustrations.
Other good references include early wood carvings, some of these show the outer layers of skin being pulled away to reveal internal organs and musculature.
If you have the stomach for it, there are also places you can study the actual human body as it decays both in person and through pictures on sites dedicated to forensic students. These can and will get very gruesome, which is usually what you want to portray with the undead, but only you know your limits on what you can handle.
For your diseased corpses there are many sources for maladies and frightening deformations often with illustrations and medical studies. Other especially disturbing examples can be found in wax sculpture and even preserved remains. Some truly fascinating subjects can be explored in the direction of diseased or deformed corpses. Consider resources llike the Musee Dupruytren, which offers a large collection of medical curiosities.
These can also be hard to look at but when utilized properly, they can add a sickening level of detail to your walking dead that will surely frighten your viewers. Games like Bioshock have used this particular technique to great success with their splicers and other
denizens of Rapture.
Finally an especially useful tool to any artist is a 3D model of the human skeleton. Not only will this help with your undead, but it is a good reference for any illustrations involving people. There are places which offer posable skeletons, models which have removable muscles, and there are even models which have both. You may also want to get skeletons of various animals as well as several different skulls, there are also places which offer odd skeletons like ones with deformaties, or even creatures like deomons, centaurs or the hulk; all you have to do is look.
The more believable your undead creatures seem, the more disturbing they will be. If it seems like something that may well creep out of the darkness and devour you, you will have succeeded. So, in the name of art, start digging up bodies and study study; if you can perform necromancy you can do movement studies too, all the better. Get out there and research your undead anatomy!
(Disclaimer: Don’t actually dig up bodies to research decomposition; that is vandalism and that is bad. Do not animate said vandalised bodies; that will cause a whole other problem which is also bad and would be avoided if you heed the first warning. Bioshock is a copywrite of Irrational Games.)