How do I add a layer in G'MICol online

How do I add another layer on the online GMIC site? Whenever I select the Style Transfer tab, it gives me options for which style I want to use. One option is a custom layer, but there doesn’t seem to be any real way to import another image.

That site is more so for demonstration, but it’s currently incomplete. You’d be better of with using a plugin in your favorite image editor.