Just replied to a call for illustration references from the Illustration group on the WAE Network site (Writers, Agents & Editors Network) and thought you might like the info as well.

I have a ton of references and would like to share some of the best ones, here:

Joseph Sheppard's "Bringing Textures to Life"
James Gurney's "Imaginative Realism"

Picture book illustrators: Keith Baker, Yuyi Morales, Jim LaMarche, and Carol Heyer (especially her "Humphrey" books).

For less recent illustration inspiration, I look to Elsa Beskow, John Bauer, Tove Jansson, N.C. Wyeth, the d'Aulaires, Wanda Gag, and Ivan Bilibin.

Fine artists:  Duane Keiser, Karen Jurick, Jeffrey T. Larson, Carl Larsson, Mary Cassatt, Alphonse Mucha, John W. Waterhouse, Degas, Gauguin, Lautrec, Klimt, Picasso's minotaurs and doves, and a Finnish artist named Askel Gallen-Kallela.

YouTube tutorials for painting: http://youtube.com  (search for the media you use, there are too many to list)

Tutorials for Photoshop: http://psd.tutsplus.com/category/tutorials/illustration/

Tutorials for Illustrator: http://vector.tutsplus.com/category/tutorials/illustration/

Other online inspiration: http://muddycolors.blogspot.com/

And, last but certainly not least, the Library of Congress' Prints and Photographs Collection for historical reference: http://www.loc.gov/pictures/

Hope these fire some inspiration!