You are able to delete a docker image using the Container Service user interface. This will delete all Commands associated with the image and delete the image data from your Compute Backend if it has been pulled locally.
In the XNAT UI, select Administer → Plugin Settings
On the Plugin Settings page, choose Images & Commands. This screen is organized by docker image, with a table under each image listing the Commands that use that image.
Find the section for the image you wish to delete. Click the Delete Image button in that section.
A Confirmation dialog will appear. Click OK to delete the image and all its Commands, or click Cancel or the X button in the corner to not delete the image and its Commands.