You can use either an image or a gif as background for your wall. If you’d like to have just a picture, you can simply upload it under Settings > Design after clicking "Customize color scheme" 👇
You can check out in this article the recommended resolution for background images.
But if you prefer to have a gif, add the following CSS code with your gif to your custom CSS box under Settings > Design 👇
background-image: url(https://media.giphy.com/media/qNpqSmKO4bn68/source.gif); /*put your gif here*/
In case you have further questions, don't hesitate to contact the support team via chat or firstname.lastname@example.org!