A few month ago while I was searching on forums for references I landed on Hubert's Website, and I have to say I got shocked by his work.
YOU HAVE TO CHECK IT OUT

Hubert is doing some amazing paintings, and more than that he's sharing a lot of tutorials about his technics and step by step process. Sorry the site isn't in English, but still it's worth watching.

here is some direct links to a selection of tutorials:

La nuit
Barbe Bleue

and an extra link that many Star Wars fans will like ( French/English version) :

XWing