Some of the answers here are for inifite page. What Salman is asking is lazy loading of images.
Plugin: http://www.appelsiini.net/projects/lazyload
Demo: http://www.appelsiini.net/projects/lazyload/enabled.html
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
answered Feb 25 '11 at 13:18
Alec Smart