With this change, the return value of org-string-display does not really
make sense for images, but at least the function doesn't barf on complex
but valid display properties.  WDYT?

Thanks,

-- 
Basil

Emacs  : GNU Emacs 27.0.50 (build 3, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2018-09-01
Package: Org mode version 9.1.9 (release_9.1.9-65-g5e4542 @ /home/blc/.local/share/emacs/27.0.50/lisp/org/)