How can image HTTP requests be minimised?

CSS Sprites are a means of combining multiple images into a single image file for use on a website, to help with performance.

Here’s an example sprite, with three different countries flags combined into a single image:

You set the same background-image on several CSS classes and set the background position and dimensions of the individual classes to display a single portion of the sprite. Here’s some code that demonstrates the concept:

.flags-canada, .flags-mexico, .flags-usa {
  background-image: url('../images/flags.png');
  background-repeat: no-repeat;
}

.flags-canada {
  height: 128px;
  background-position: -5px -5px;
}

.flags-usa {
  height: 135px;
  background-position: -5px -143px;
}

.flags-mexico {
  height: 147px;
  background-position: -5px -288px;
}

Source: https://css-tricks.com/css-sprites/