/usr/lib/x86_64-linux-gnu/libfltk_images.so is in libfltk1.3-dev 1.3.2-4.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libfltk_images.so.1.3
/usr/lib/x86_64-linux-gnu/libfltk_images.so.1.3