This value tells browsers to make the image repeat across the page and down the page to produce a grid effect. Delete that statement if you don't want your GIF to create a tiled background.

Personalise your Safari by adding an image of your choice to have in the background of the Start page in the new Safari 14 ...

What others have said. For JPEGs just cut out the background, create a layer behind it with the colour or texture of your website and save in the format of your choice.