A free app for Android, by HappyDay Studi0.

Find the Difference – Spot Fun is a game where you need to spot the difference between the two pictures, and the more you do, the more you'll enjoy it.

The game consists of more than 1000 levels, and you can play them all without having to pay anything. 

The difference you need to find will be displayed at the bottom of the screen, and you'll have to click on it to see what the picture on the left or the right side of the screen looks like. Then you need to focus on the details to find out what the pictures really look like.