image "slug": null

[jalleyne]

When using the api to generate snapshots, the name does not translate to a slug like the default (digitalocean) images. This breaks tools like docker machine (see issue: docker/machine#3086).

[franklouwers]

You need to use the image id. As per the docs (my emphasis in bold):

A uniquely identifying string that is associated with each of the DigitalOcean-provided public images. These can be used to reference a public image as an alternative to the numeric id.

So only the public images have a slug.