If you are a photographer, you'll find many intriguing subjects in Chichicastenango. If you want to take pictures of the locals, it is ok to do so from a distance that does not intrude upon their personal space.
If you find someone that you would like to take a close-up of, however, you need to tip them about 5 quetzales. The reason for this (supposedly) is that they feel you are stealing their animal spirit when you capture their image close up. 5 quetzales is less than $1 US, so its not a big deal -- I've gotten some great photographs this way.