So, I'm not a historian, but I understand that there were multiple technologies available before silver gelatine took over in the 20th century. There was gum bichromate and dagurrotype and probably others.
I was reading the silver gelatine emulsion making forum and it looks fun, but it looks hard.
After the zombie apocalypse, industry as we know it will not exist, so photography will have to be something pre-industrial. What is the easiest chemistry, period? Requirements are a long life (decades or centuries). Did technologies develop roughly from simple to complex and from easy to hard? Now, knowing the history, what is the easiest way to get a fixed image using the simplest equipment?