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