Some of the answers here are for infinite page. What Salman is asking is lazy loading of images.
Plugin
Demo
EDIT: How do these plugins work?
This is a simplified explanation:
- Find window size and find the position of all images and their sizes
- If the image is not within the window size, replace it with a placeholder of same size
- When user scrolls down, and position of image < scroll + window height, the image is loaded