Using an animated GIF as the background of a website can be done using CSS. Here's a step-by-step guide on how to do it:

Upload your GIF: First, you need to upload your GIF to your website server or an image hosting service. Once uploaded, get the URL of the GIF.

Set the GIF as a background image: You can use CSS to set the GIF as the background of an element or the whole website. Here's an example on how to set the GIF as the background of the whole website:

body {

  background-image: url('path_to_your_gif.gif');

  background-repeat: no-repeat;

  background-size: cover;

}

In the url('path_to_your_gif.gif'), replace 'path_to_your_gif.gif' with the URL of your GIF.

This will set the GIF as the background image of your website, it will not repeat and it will cover the whole background.

If you want your GIF to repeat (tile) across the background, you can use background-repeat: repeat;.

If you want the GIF to be fixed as you scroll down the page, you can use background-attachment: fixed;.

If you want the GIF to automatically resize according to the viewport, you can use background-size: 100% 100%; instead of background-size: cover;.

Remember, while using animated GIFs can be a fun way to add some life to your website, they can also be distracting and potentially slow down your website's load time. So, use them sparingly and ensure they enhance the user experience rather than detract from it. It may be a good idea to compress the GIF before uploading.