You said you had an LCD showing black boxes.
I don't see an LCD in either of those photos.
Where is the LCD?
You need to show how the backpack is connected to the LCD.
You showed the solder connections of the header to the backpack and the solder connections of the PCF8574 chip to the backpack (great photos!) but you didn't show the solder connections on the LCD module.
I'm assuming you either have a female header soldered to the LCD that the backpack plugs into or a 16 pin male header on the LCD and the backpack and the LCD are both plugged into a bread board.
You need to show the solder connections on the actual LCD and how the LCD is connected to the backpack.
--- bill