Spanish for "altar piece", retablos are a tradition in folk art where
religious scenes or images are painted on inexpensive material
(for example, wood, or thin tin and iron sheets),
or crafted into small ornamental wooden boxes placed about the home
or church. The practice is of European origin, and was transplanted to the
western hemisphere by the Spanish conquistadores. These icons are meant as a form of religious devotion, or, as a remnant of pre-conquest
traditions, as good luck symbols.
Several themes are common in retablo art, with the crucifixion of Christ,
The Virgin Mary and Child, and Virgin of Guadalupe being
particularly common, along with a host of saints and apparitions. They
may also express gratitude for being cured of illness or worry by religious
devotion.
Retablo boxes are found throughout Central and
South America and are often very finely crafted, with doors opening
onto a richly detailed scene.
The paintings on tin are indigenous to Mexico, where the raw tin sheets
were common and inexpensive. The practice of hand-crafting retablo paintings
faded at the end of the nineteenth century when the inexpensive mass production
of pictures and engravings became possible.
A collection of Mexican retablo paintings can be found at
http://www.nmsu.edu/~artdept/retablo-web/, from which some information for this writeup was taken. The New Mexico State University art museum has an extensive collection of them as well
if you happen to be in the Las Cruces area.