On the page properties there is a tab for the thumbnail image. I added an additional tab for a background image.
But it seems to be tricky to have multiple image tabs in a dialog.
For the classic/widget based UI I had to explicitly add 'resourceType = foundation/components/image' to the new background image field so it is stored correctly. But now it works for me.
Now I also wanted to add the background image to the touch UI. So I copied the existing definition of the thumbnail image from the granite dialog and added it on a new tab. But if I open the page properties in the touch UI and add a new image on the thumbnail or background tab, both tabs always show the same image. In CRX I see that the nodes and properties are mixed up between both image nodes.