Here's another site that has a good collection of photos of Mexicans, including Dorothea Lange, at the Library of Congress. It's called the Farm Security Administration-Office of War Information Collection and there are lots of interesting images of Mexicans. Same criteria, do a search for Mexicans and you will come up with many hits, mostly in Tejas and El Paso. Here's the link,
David O. García
García's own semiotic site is here.