This is a contribution from Moko (cleaned up by Pip) of a postcard being sold on Ebay. It not only illustrates how tribal people in remote areas may have less inhibitions about nudity but how photographers from so-called civilized areas might exploit this for monetary gain or fame in the name of scientific ethnography. After all, this did not appear in some text in an academic tome, but put on a postcard that could be disseminated publicly. Nothing is really known about the provenance of this image but the seller claims it is over 100 years old.
To my eye (correct me if I am wrong) these girls are not Han Chinese—the dominant population in China—so the photographer was most likely English but there is a remote chance that he/she was Chinese.