Thanks for the photos and for getting back to us.
Wire labels such as BK indicate an incomatible system.
These systems usually only work with thermostats made by the same manufacturer. But some HVAC systems may also have the option to use standard heating system wires. Contact a local pro and have them check if your system can be rewired to be Nest compatible.
Just one quick final check in here since activity has slowed down. We'll be locking the thread in the next 24 hours, but if you still need help, I would be happy to keep it open. If there's more we can do, just let me know.