Here is the full list of all 10 A Mysterious Gallery achievements.
You have completed the tutorial.
You have used the revealing magnifier 20 times.
You have found a secret code.
You have unlocked a painting.
You have unlocked half the paintings.
You have unlocked all the paintings.
You have found 20 hidden items.
You have placed 20 items.
You have visited the gallery.
You have found the Easter egg.