(I kept the files as big as possible, so you can see some good details)
First off: Hieronymus Bosch. You can find all about him here
He was a Dutch painter who lived from 1450 till 1516. His works are super famous and far ahead of his time. This stuff is very dark and there are tons of details that you will miss in these files. This is what the Warp looks like....go read about him, it will inspire you to model.
This last one reminded me of the Emperor on his throne...
The other artist I really like is Gustave Dore, a French illustrator from the 19th century who is famous for his biblical drawings. Find out more here. I have an old copy of Dantes Divine Comedy (click here for more info) and the illustrations that Dore did for that book are narly and scary and seem exactly like the world Horus and Fulgrim were envisioning when they betrayed the emperor. His visions of Hell give me the creeps and are extremely facinating. Some of his images would look very nice as a banner in a chaos army.
If you think this was fun, hunt down some books about these artists and study their work. You will be amazed!
Hope this was fun!