How to delete a docker image

Question

A Dockerfile is included in my project to create an image. I've tweaked that Dockerfile and want to verify whether it still works. So I quit my program and used docker image ls -a in Powershell to list all of my Docker images. Using docker rmi -f $(docker images -a -q) and docker image prune -a and  I deleted all unnecessary images.

However, instead of being erased, the image is merely 'untagged':

Untagged: my_image:latest
Untagged: my_image@sha256:16eb92a476be0...

Before attempting to erase the picture, all containers are stopped and removed.
When I restart my application to create a new image, it continues to use the old one:

> docker image ls -a
REPOSITORY                     TAG           IMAGE ID            CREATED      
/my_image                      latest        0d79904a74b0        1 months ago

How can I physically delete the old image so that my app can create a fresh one?