The picture on the right looks a little strange, some jumbled monkeys there. It will be worth your time to fixate it for at least 10 seconds. Let your gaze “hang” on the tiny red target at the centre, making ready to move your mouse over the image thereafter. On the white background you may recognise a face, a topic of 2009.
Alternatively: After fixation, close your eyes. After a few seconds a face will appear.
Faces are difficult to recognise when rendered as a negative. Prolonged fixation creates a retinal afterimage, which over several seconds is a negative afterimage, making it recognisable.
The longer and the more steady you fixate, the longer lasts the afterimage. When it fades, blink and it will be prolonged.
On of the tricks is to introduce the tiny white lines (here chosen to delineate monkeys, appropriately). These impede face recognition in the initial negative (related to the blocking illusion). In the afterimage, however, they are gone because an afterimage is always a little blurry.
Rob Jenkins & Richard Wiseman give some delightful background and also the original image on their website referenced below.
There are a number of similar images floating around the web.
Jenkins & Wiseman (2009) on the Wiseman website
Thank you, Rob & Richard, for offering this delightful composition.
Thanks to John for suggesting the eye-closure approach.