Watchdog displays a harmless image, but when touching it an angry dog starts barking as scary as it gets.
You can replace the first image with any photo of your photo library – a funny idea is to use a screenshot of the Home Screen of your iPhone.
Surprise your friends that can not get their fingers off of your iPhone!
You can make screenshots at any time by quickly pressing and releasing the Sleep/Wake and Home buttons at the same time.
For best effect make sure to switch off the silent switch of your iPhone and turn up the volume.
Note that the author of Watchdog can not be made responsible for dropped devices.
Related Blog Post: Being Rejected. Not.