There are two sites that provide general information about Posada, one in English and one in Spanish:
in English:
http://muertos.palomar.edu/posdad.htm
in Spanish
http://www.arts-history.mx/posada/posada.html

Arizona State University has a wonderful site that discusses in great detail one of Posada's most important prints, Calavera Revolucionaria.
The site discusses this particular print in the context of Posada's life and the turbulent history of the time.
http://mati.eas.asu.edu:8421/~getty/html_pages/posada9.html

Excellent Site on Posada by the University of Hawaii
http://www2.hawaii.edu/~speccoll/posada.html

José Guadalupe Posada
The Peoples' Artist
Many of the images associated with Día de los Muertos are based upon the work of Posada. José Guadalupe Posada died more than a hundred years ago, but his artwork is still very popular. Each year Posada's skeletons are seen in thousands of different forms from puppets to paper banners.