In the Admin Menu, click on the Content link.
From the Content list, click the Media tab.
Click the Add Media button.
You'll be redirected to a page of links to different media types.
Click the Image link.
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.
The completed image will appear within its entity.
Note the dimensions that automatically appeared.
Click the thumbnail image to view the original sized image.
Revisiting the media list at Admin menu > Content > Media tab, you'll find the test media appears in the list.