There are many unspoken rules regarding etiquette when attending an Alms Giving Ceremony, firstly shoulders, chests and legs must be covered in modest clothing as a mark of respect. You should also remain at a suitable distance from the monks and do not interrupt the procession under any circumstances. Therefore photographs may be taken, but from a distance and never use a camera flash.
When seated, shoes and socks must be removed with your feet tucked underneath as you observe the ceremony in absolute silence. Female attendees must keep their head lower than the monks when giving alms and they must not talk to or touch the monks at any time, even when making an offering. Suitable offerings include some simple food which you can probably arrange at your hotel or buy from a local market en-route, if you buy alms on the street do not negotiate on price as this is also considered highly disrespectful.