Attempting to upload some new photos between around 3 to 5MB, the majority of the uploads failed, and this error was occurring repeatedly in my nginx error.log:
2018/08/06 07:18:37 [error] 43#0: *5 client intended to send too large body: 3125530 bytes
Based on this tip from here, the solution was to increase the default POST body size config with this setting in my nginx.conf:
client_max_body_size 5M;