NETem emulates a network link, typically a WAN link. It supports bandwidth limitation, delay, jitter and packet loss. All this functionality is already build in the linux kernel, NETem is just a menu system to make the configuration user-friendly.
For more informations see http://www.bernhard-ehlers.de/projects/netem/index.html.
The image is downloadable from the releases area.
If you want to build it yourself, you need a linux system and install packer (https://www.packer.io) and qemu.
Then build the image with
rm -rf output-qemu packer build NETem.json