Use the following steps to view the hidden photos in the background of some levels.

Books: Tap on the peeled wallpaper in the background on the far right side of level 3-19.
Candy beaker: Tap on the peeled wallpaper above the second star in level 2-19.
Computer screen: "Like" the "Cut The Rope" Facebook group through the link in the Professor's Album.
Magnifying glass: Tap the peeled wallpaper in the background on the top left side of level 1-14.