A disk image file is just that - an 'image' of the completed DVD. You must use Apple Disk Utility (or Roxio's Toast) to burn it to writable media so that it remains in UDF format. If you simply burn the image to writable media from the finder, you create a Mac OS Extended format disc that won't play in DVD players.
If you have burned a disk image to writable media and it doesn't play in a DVD player, insert it in your Mac, click on the icon and do a 'Get Info'. I'll bet the format shown is Mac OS Extended and NOT UDF as it should be.
F Shippey