How to Create an image in DeployStudio
The .nbi is the NetBoot set, it's the disk you are booting the Mac you want to image from that contains DeployStudio Runtime. From there you will run a workflow, that can incorporate an image if you want. The workflow is created in DeployStudio Admin.

To create images, I like to use AutoDMG: https://github.com/MagerValp/AutoDMG

Once you get your image, you'll want to make sure it's named with a .hfs.dmg (AutoDMG will take care of this for you).

You copy the image on to your DeployStudio repo, by mounting the file share and copying the file into a folder called Masters and a subfolder inside that called HFS (I'm doing this from memory so that name may be slightly off).

Once your Master is in place, you should be able to select it from the Imaging step in a new workflow you create in DeployStudio admin.

Make sure the fileshare and its contents are accessible by the user you setup for DeployStudio... that may be the issue.