Hello,

This is not a default option but this is a workaround:
You can set the page’s background to transparent in the page’s General Settings > Page’s Options & Style > Page Content Background, check “Transparent” option.
Then add to the page’s custom css box this css:

#content {
    background-image: url(https://codeless.co/specular/default/wp-content/uploads/2014/07/1231-compressed.jpg);
}

change the image url to your choice.

Best regards!

Hello,

Please look for the gear icon at the top-right of option panel. See screenshot.
Let us know.

Best regards!