Apparently this was a known bug in ImageMagick 6.9.9-15 (and 6.9.9-17) (https://github.com/ImageMagick/ImageMagick/issues/825). It has been fixed in version 6.9.9-18.
- bug#28692: Emacs fails to display images Diego Nicola Barbato
- bug#28692: Emacs fails to display images Ludovic Courtès
- bug#28692: Emacs fails to display images Diego Nicola Barbato
- bug#28692: Emacs fails to display images Diego Nicola Barbato
- bug#28692: Emacs fails to display images Marius Bakke
- bug#28692: Emacs fails to display images Diego Nicola Barbato
- bug#28692: Emacs fails to display im... Ludovic Courtès