Tried background image here, it did not show up? I was using windowed mode, then switched to full screen, just for testing purposes, the same there, no background image. The image is in the metadata directory, theoretically it should work. Do I have to activate any other important/secret parameter?
UPDATE: Found the error reason. If 'sheet music' is set to 'Off' it seems background images do not show up. Just set sheet music to on and the background image was there.
UPDATE: Interesting behaviour, only the first time if 'sheet music' is 'Off' background images are not shown. If you do 'sheet music' = 'On', they show up, interestingly if you do then 'sheet music' = 'Off', they still show correctly. For testing I closed Synthesia, restarted, same behaviour as explained here. So the first time it never works if 'sheet music' is 'Off'. This seems to be a minor bug or so.
UPDATE: It is not necessary to play even Synthesia once, just clicking once on 'sheet music', then back on 'Off' seems doing the trick. But without this extra click work the background image will not show up, at least here. Later it can look as below, for my test.

- background.jpg (259.62 KiB) Viewed 15663 times