While only public images have “slugs,” all images have an ID. You can list all of your snapshots and backups using:

doctl compute image list-user

The output will look like:

ID         Name          Type            Distribution    Slug    Public    Min Disk
5812605     foobar         snapshot    Ubuntu                       false       30

This will provide the ID for the image as well. From there, you can create as normal using the image’s ID for the --image argument. E.g.:

doctl compute droplet create --region nyc3 --image 5812605 --size 8gb foobar