Painting on glass is a popular Polish form of folk art by which the artist paints a picture on the reverse side of a glass surface.
In Poland, the szopka is a portable Christmas crib or manger scene carried by young boys around their village or neighborhood.  In exchange for the caroling and entertainment they provide, the boys receive a few coins or, even better, some tasty holiday treats from their hosts.  After going from house to house in their own heighborhood, the sometimes parade into other sections of town or even different villages.
By far the most elaborate szopkas are those from the city of Krakow.  Modeled after the great Mariacki (St. Mary's) basilica in Krakow's main square, this form of szopka is decorated with brightly colored foils.  
Painting the Szopka Krakowska on the reverse side of glass is a specialty of master artist Janina Oleksy-Lew.  All pictures vary.  No two alike.