Here are a couple of ideas.
Make sure the geometry of file.png matches (exactly) the geometry and color depth of bg.png. Also if I remember correctly size does matter. There is a maximum total byte count of this background image.
Secondly some uefi firmware can’t / doesn’t display background images. There is nothing you can do it make it work, its flaky uefi firmware.
As for the uefi boot manager. Yes I’ve seen that myself. I think one of the developers was going to look into that issue. I don’t know if the fix will be in 1.5.6 or 1.5.7 if they can work it out. One of the issues we’ve seen is some people want windows to be the first boot option after imaging and other want to keep what ever boot order they currently have and not have FOG change the boot order.