https://docs.magento.com/m2/ce/user_guide/cms/editor-insert-image.html

Inserting an Image

From the editor, you can insert an image that has been uploaded to Media Storage, or link to an image that resides on another server.


Media Storage
OpenMethod 1: Insert an Image from Media Storage

1. If necessary, tap  Show / Hide Editor to work directly with the code.
2. Position the cursor where you want the code for the image to be inserted. Then, tap  Insert Image.
3. Choose the image that you want to use, and tap  Insert File .
4. To view the image in WYSIWYG mode, tap  Show / Hide Editor  again.
OpenMethod 2: Insert an Image from Another Server

Use this method to insert an image that is available online, but resides on another server. You must have the full URL of the image to complete the process.

1. If necessary, tap  Show / Hide Editor  to work in WYSIWYG mode.
2. Position your cursor where you want the image to appear.
3. On the Editor toolbar, tap the  Insert Image  button. Then, do the following:

Insert Image Button
a. In the Image URL field, paste the full URL to the image on the other server.
b. In the Image Description field, enter a brief description of the image.
c. In the Title field, enter a relevant title for the image.
4. Tap  Insert  to complete the process.