User Level: 
Click on an image to make it bigger.
Drupal 8 - Media - Image Media Type - Add Test Media

1. Add Media

In the Admin Menu, click on the Content link.

From the Content list, click the Media tab.

Click the Add Media button.

Drupal 8 - Media - Image Media Type - Choose Image

2. Select Image Media Type

You'll be redirected to a page of links to different media types.

Click the Image link.

Drupal 8 - Media - Image Media Type - Test Image

3. Add Image Information

On the Add Image form:

Name field = enter the name of the image

Image upload field = browse to and upload the image of your choice

Alternative Text field = add some alt text

Do not enter any values in the Width or Height fields.

Click the Save button.

Drupal 8 - Media - Image Media Type - Completed Test Image

4. Completed Image Addition

The completed image will appear within its entity.

Note the dimensions that automatically appeared.

Drupal 8 - Media - Image Media Type - Original Image

5. View Original Sized Image

Click the thumbnail image to view the original sized image.

Drupal 8 - Media - Image Media Type - Media Added to List

6. View Entry in Media List

Revisiting the media list at Admin menu > Content > Media tab, you'll find the test media appears in the list.