When you next run toolbox, you should see it pull down the requested image.
$ toolbox Pulling repository debian 835c4d274060: Download complete 511136ea3c5a: Download complete 16386e29a1f4: Download complete Status: Downloaded newer image for debian:jessie core-debian-jessie Spawning container core-debian-jessie on /var/lib/toolbox/core-debian-jessie. Press ^] three times within 1s to kill container. root@myserver:~#
It's that simple.