After playing this game for a while, I believe that spin is very important. Saving your gems for spins is the best. Using gems spin 1 time everyday if you can. The spin has a trick. If you close your browser before the spin stops, this time won't count and you can do it again. So try to aim multiplier.
Try to unlock CoinAppocalipse as fast as you can. It helps you obtain keys much more easier. I get about 10 keys for an hour if I use autoclick.
Use gems for reply is not worth imo. I played this game about 15 day, and unlocked over 90 pics with only 2 replies.