Can someone on Google Earth get someone's address from their FB photo of their back garden lawn without the front door number/address?

yes, if you have metadata (EFIX), which gives GPS cordinates, geotagging also, also if you have identifiable landmarks, you can do a reverse image search, also use social media clues, look these up, google all these things or google your question, try it

i dont htink so unless theres a previous photo of the back garden that is public domain and specifies the location? (from Google)

Only the Chinese Government.