Here is the full list of all 10 A Mysterious Gallery achievements.
You have used the revealing magnifier 20 times.
You have visited the gallery.
You have completed the tutorial.
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 found the Easter egg.