You can add some CSS to your 'body' tag like this:
<body style="background-image: url("image.jpg")">
Or prefferably if you have an external CSS file linked to your HTML web page, you can add this to it:
'image.jpg' is just a dummy file name which you should replace with your own file name. Also, if your file is inside a folder i.e. 'images' then the url should be "images/image.jpg" assuming your html file is in the same folder as your 'images' folder. Also, remember to keep all filenames, folder names and html tags in lowercase letters.