Pillow, if you installed pip directly on bundled python



The folder where the bundled python interpreter is also contains pip, if you installed it with get-pip.
To get pillow, which is obscurely included in python3 in many Linux-distros but not all, you need to use pip properly.

Note that in my setup I have blender installed in a user directory. Thats why I can install both pip and pillow without root-access. If you have another setup it might be necessary to use sudo …