I added a style to the HTML BODY tag, but it does not work

Using the BODY tag to assign an image background or other background to a page is unfortunately not a good idea when preparing an HTML email message. That's because many email clients will ignore that style definition.

To add a background to a page or a large section of the page, it's much better to revert to the "old days" of using HTML tables as a layout element, and then to assign a background to the table (directly in the <table> tag). To have the background fill up the screen, make sure to remember to set the width of the table to 100%.