A couple things. First, it's best to add styling via scripting, since the rich-text editor will delete it otherwise and the plain text editor adds unnecessary newline tags. With scripting enabled, that's accessed via the := button next to the page's title. Second, it's better to use CSS. Third, you need to link directly to the image. Your link just goes to an HTML page containing the image:
$PAGETEXT := "<!--<style>
body {
background: url('https://i.imgur.com/oUOvSlM.jpg');
}
</style>-->" + $PAGETEXT
