To add value to @diomed answer, a sprite image is an image which comprises multiple images, this is done so to avoid multiple http requests to the server that hosts the image. Although the file size might not change much, if at all, the fact that you are making a single request helps with the loading times and the rendering (or pre-rendering) of the webpage.
Rendering is the process with which the website is “drawn”, pre-rendering is the same, but done before the user can actually use the website (hence the use of loaders while you wait for the website to load).
Sprites might become obsolete with the coming of the HTTP2 protocol, but for now they are a good choice. The deal with sprites is that you can use CSS to show a particular part of the sprite image.
One image could be at 5px, 5px while the other one could be at 20px, 20px. Changing the position property of the sprite image in css will change the displayed part of the sprite image.
Hope this can help