This portrait resembles a high school graduation portrait. According to her Wikipedia article, María Cristina was born in 1833 so this portrait of a 16- to 18-year old María Cristina would be from around 1850. Her mother was Luisa Carlota whose mother was María Isabela. Her paternal grandmother was María Luisa de Parma.
From the lost gallery's photostream on flickr; spots and flaws throughout image removed with Photoshop by gogm. I made the image lighter; this brought out considerable detail. Width set at 29 cm.