Created attachment 1910 Suggested solution. The Configure dialog of the splash engine 'Simple' has a 'File open' button for a custom background image. The opened dialog for this button has a wrong ordering of the 'Cancel' and 'Ok' buttons. Usually first comes 'Cancel' and then 'Ok'. Attached patch corrects the ordering.
Ended up just changing it to use a GtkFileChooserButton instead of the entry/button combo. Buttons in the chooser are set up automatically now.
Great.