Captions should accompany all images included in the body of an academic paper. A caption includes much of the same attributive information that a citation provides for textual sources. In attaching a figure number to the image, a caption also allows you to refer easily to that image from any point in your text without resorting to location dependent descriptions (e.g., "the image on page 4," "the image above").
In general, captions for artworks will take the following form:
Figure number Creator, Title of Work, date created, medium on support, measurements. Collection or Museum, City of Collection or Museum (your means of obtaining the image)
In practice, individual elements may frequently be missing or not fully certain, as in the following examples:
Precise date unknown:
1 Norizane, Netsuke of a Seated Puppy with a Short Curled Tail, first half of 19th century, black and gold lacquer on wood, 1 1/8 x 1 x 1 1/2 in. (2.9 x 2.5 x 3.8 cm). Metropolitan Museum of Art, New York (accessed in Artstor)
Artist unknown, precise date unknown:
2 Scholar Walking with a Staff in a Landscape, 18th-19th century, ink and color on silk, leaf with painting: 11 13/16 x 12 5/8 in. (30 x 32.1 cm). Saint Louis Art Museum, Saint Louis, Missouri (accessed in Artstor)
Artist attributed but not known with certainty, approximate date known:
3 Attributed to Orazio Gentileschi, St. Simon and St. Jude Baptising the King of Persia and His Generals, ca. 1594, pen and brown ink with gray-brown wash over black chalk, 416 x 284 mm. British Museum, London (accessed in Artstor)