In a fresh 7.6.15 installation on a Ubuntu 16 VM with the introduction package, images and their thumbnails are not displayed. GraphicsMagick is installed. GFX configuration is modified with the install tool per the wiki document. No php errors listed in php_errors.log. Images are listed in Filelist.
The path of GraphicsMagick in the installtool might be wrong
[GFX][im_path]
[GFX][im_path_lzw]
On my Ubuntu server the path is /usr/bin/
Other common value is /usr/local/bin/