I found a really good source on google books, called 'The Expanding Discourse: Feminism and Art in History' by Norma Broude and Mary D. Garrard. It has a fairly in depth description of the painting, and mentions that the dress depicted is the traditional clothing of Zapotec women. Apparently Zapotec women "represent an ideal of freedom and economic independance" to a large extent even when compared to other indigineous Mexican groups. The book also mentions that Kahlo more often used Aztec imagery, and that it was an unusual choice when compared with her other work. I'm not certain what this says about her thoughts on New York and Mexico. Did she feel that she had more freedom in Mexico?