Look at it this way. Even a Thai citizen must produce their original Thai ID card on request. They cannot produce a copy. If anyone of authority request your ID documents (passport) then you must be able to show them the original document and not a copy, but saying that it is up to the person of authority if they will accept a photo copy or not. Many foreigners take the risk by copying their passport information page, plus their visa, extension, entry stamp, departure card and either laminate the copy or some of them even reduce the size down to a credit card size for ease of carrying. But at the end of the day it is up to the authority on what they will accept. It is up to you if you want to take the risk but everyone in Thailand including Thai citizens must carry their original ID documents.