La Divina Pastora: Virgin Mary as Shepherdess
The article I was reading was A Short History of Rosaries in the Andes by Penelope Dransart. She says:
"One of the most prominent attributes of the Rosario [image of the Virgin as Our Lady of the Rosary] is, of course, the rosary, but this feature is also shared in the iconography of a related version, the Virgin of the Shepherds. In this respect, a painting dated 1703, entitled La Divina Pastora (The Divine Shepherdess) in the Museo Nactional del Arte, La Paz, Bolivia, is of interest. In this picture the Virgin is surrounded by sheep which have roses in their mouths and rosaries of black beads hanging over their backs. Alternatively, some of the sheep have the characteristic knotted belt cords which form part of a Franciscan monk's habit draped over their backs. In the background, a lamb wearing a black rosary with a pendant crucifix has a speech scroll emanating from his mouth, bearing the words. 'Ave Maria.'"
I would love to see what this picture looks like, but the La Paz museum doesn't seem to have much of their collection online, and other sources have come up blank so far. I did, however, run across this one, which seems from the description to be along the same lines, although without the black rosaries. This is a modern painting in the "Baroque Colonial" Peruvian style by David Chavez Galdos:
In Spain, devotion to La Divina Pastora appears to have started in Seville, also right around 1700, and may indeed have originated there and been carried quickly to the Americas. Brother Isidore, a Capuchin (Franciscan) priest greatly devoted to Mary, is said to have commissioned a painting of the Virgin as shepherdess from the artistic school of Alonso Miguel de Tovar. There are earlier references to Mary as shepherdess in the writings of Saint John of God, Saint Peter of Alcantara and the visionary Maria de Agreda.
The Spanish Wikipedia article (whose sources include two books on the devotion) recounts that Brother Isidore specified that the Virgin wear a red tunic, a white sheepskin around her waist, and a blue mantle slung across her left shoulder. A shepherd's staff is behind her on the right. In her left arm she holds the Infant Jesus and her right hand reaches toward a sheep taking refuge in her lap. She is surrounded by sheep bearing rose garlands in their mouths.
A little investigating online suggests that devotion to La Divina Pastora spread widely in the early 1700s and is still popular in Spain, Portugal, South America and the Philippines. The connection to roses and the rosary is not always apparent, as in this statue, located in Nava del Rey (Valladolid, Spain) and attributed to the woodcarver Luis Salvador Carmona (1709-1767). There are a number of Brotherhoods or Societies of the Divine Shepherdess in Spain and Portugal.
Variations on this title of Mary include Divina Pastora de las Almas (Divine Shepherdess of Souls), Madre Divina Pastora (Divine Mother Shepherdess) or Madre del Buen Pastor (Mother of the Good Shepherd, since Jesus is often given that title). The images may also vary: Mary is usually wearing a big floppy hat, but she may be shown alone or with the Infant Jesus, with or without a shepherd's crook. She may have a whole flock of sheep or just one or two. The roses are sometimes red on one side and white on the other: this may have connections with earlier rosary images that show groups of white, red and gold roses representing the Joyful, Sorrowful and Glorious mysteries of the rosary.
As with other images of Mary -- and as you'll quickly see if you Google images of "La Divina Pastora" -- many decorations can be added to the original statue or painting, including rosaries over Mary's arm, lace and ruffles on her dress (a bit impractical for an actual shepherd!) and a gold crown, either instead of or rather awkwardly perched on top of her hat. The painting above solves the hat + crown problem by having cherubs hold the crown in the air over her head.
The Virgin Mary has a seemingly endless variety of titles in the Christian tradition, each meaning something special in terms of imagery. Roses are a common metaphor for the rosary in other contexts, so it's not surprising to see them here. I'm enjoying roses in bloom where I live right now, so it seems appropriate. Whether sheep will actually eat roses in real life I have no idea!
 Dransart, P. 1998. A short history of rosaries in the Andes. In Beads and bead makers: gender, material culture and meaning (ed.) L. Sciama, 129-46. Oxford: Berg.