It is sounds like that you have seen murals that are printed on linen, not painted.
The ooriginal painting is digitalized (photographed) and then printed on raw linnen.
alternatively you can paint with very deluted acrylic paints (almost ink consistency) using technique similar to watercolor or fresco. i have done it many for event decor:
learn fresco at http://FrescoSchool.org
fresco painting video tutorials: http://www.FrescoSchool.com
Fresco School Video Channel
Contemporary Fresco Gazette
commission fresco or mural http://iLAdesigns.com
Contemporary Fresco Painting Resouce Center