"Doña Josefa Peres Maldonado offers this monument of her gratitude to the Most Holy Christ of Encino, venerated in the Church of Triana, and to the Most Holy Virgin Mary of El Pueblo, in perpetual memory of the benefit, due to her piety, that resulted from an operation that took place on 25th of April 1777, when the surgeon Don Pedro Maillé removed six cancerous tumors from her breast, in the presence of the gentlemen and ladies depicted on this canvas. Although the wound closed perfectly on the 25th of July 1777, other accidents befell her from which she died on Friday, the 5th of September, at 3 p.m., with clear signs of the patronage of the Holy Image and of her salvation." - Translation of what is written.
In thanksgiving for favors granted.
Ex Votos are often small to shoe box size paintings on tin depicting a miracle or answer to prayer, created and offered in thanksgiving to a particular saint. Such paintings are left at shrines and altars in churches - especially in Mexico and Latin America. Canon Law (Can. 1234 - 2) states that ex votos and such must be kept and maintained by the shrines.
I found a few even more 'unusual' ex votos on Tumblr ...
To see more and read the translations go here.