Creating HFS+ disk images from folders in High Sierra

In macOS 10.13, if you use Disk Utility to create a new image from a folder, it will be APFS formatted.

These images can only be read on 10.12 or newer, and there’s no option to pick a different format in the GUI.

Instead you can use hdiutil in Terminal make a folder into an HFS+ disk image:

hdiutil create -srcfolder /my/folder -fs HFS+ /my/image.dmg