Sorry if I'm being Captain Obvious here, but I was having issues publishing photos from Shotwell on my Linux desktop to my Piwigo instance
After some Googling, I discovered that I needed to make the following changes to /config/php/php-local.ini:
date.timezone = Australia/Thargominda; ***** your timezone here of course
post_max_size = 32M;
upload_max_filesize = 32M
This has fixed my problem, which was manifested as a strange protocol error during the upload. Obviously, if you upload photos that don't exceed the standard POST size, you won't have problems. For instance photos from my Samsung phone weren't an issue, but photos from my Canon 70D were.