Here is the full list of all 20 Feed All Monsters achievements.
Solve Island 1
Unlock 5 photos
Click any character three times in a row.
Feed 100 portions of food
Solve Island 2
Solve Island 3
Solve Island 4
Solve Island 5
Solve Island 6
Unlock 10 photos
Unlock 15 photos
Unlock 20 photos
Unlock 25 photos
Unlock 30 photos
Unlock 35 photos
Unlock all photos
Travel with the food bus from the Grasslands to the Snowy Mountains
Turn the music off and on 10 times in a row
Walk a combined total of 1000 tiles
Watch the credits