I just had to dig a bit more.
This is the JavaScript I added to the Node on the OnAfterFetch of the Aggregate that picked up the image:
-----------------||-----------------||-----------------||-----------------||-----------------||-----------------||-----------------||-----------------||
var styleSheet = document.createElement("style")
styleSheet.type = "text/css"
styleSheet.innerText = "body{ background-image: url('" + $parameters.ImageURL + "'); background-repeat: no-repeat;background-size: cover;}"
document.head.appendChild(styleSheet)
-----------------||-----------------||-----------------||-----------------||-----------------||-----------------||-----------------||-----------------||
$parameters.ImageURL comes from the OutsystemsUI function BinarytoURL.
So you can create the aggregate and on the OnAfterFetch you'd have something like this:
Hope this can help anyone do this exact same thing!