Introduction: While working in some applications I found that the images that are in local folder also taking time to load while opening the app for the 1st time. That means if you are setting an image as background for a div, then while opening the page for the 1st time you will find that it will take some fraction of time to set the background image.
Description: Preloading the images can help you to load the images before the page can be viewed by the user. It will cache the images in browser and will help your application for better user experience. So when the user opens the application for the 1st time he/she can view the images with out any delay. This technique is very helpful where large amount of image are used, for example image gallery or the online shopping sites.
Suppose you have some divs and you have set the image as backgroud image.
So to preload the images you can use the following CSS.
content:url(../images/content.png) url(../images/maincontent.png) display:none;
It will preload the images and cache in your browser, so when you open your application it will display the image without any dealy. You can also add as many url you need.
Summary: So preloading images will definitely help your application for better user experience and improve the performance.