The files were identified using the pngcrush software and repaired by stripping out the PNG header including the ICC profile using the command: $ pngcrush -fix -force old.png new.png; mv -f new.png old.png Each file was manually checked in an image viewer to look for any corruptions. This follows from the threads http://thread.gmane.org/gmane.games.flightgear.devel/79895 and http://thread.gmane.org/gmane.games.flightgear.devel/79898 .
Seems to be a bad commit b528caf6edb34d9df1015b47b598ec09de4774a6 in old fgdata "Reduce Z-fighting with light pole light volume by extending it below the ground" introduces a solid, non-transparent light volume.