Google Drive can be used to add images to your Workspace Description. Many other picture sources will not work.
From your browser:
Upload the image to Google Drive (+New > File Upload)
Right-click the image in your Google Drive and select “Get link”
Change permissions from “Restricted” to “Anyone with the link”
Copy only the file ID: the part of the part of the URL between
Insert the file ID into the following markdown in the Workspace description:
![<alt description>](https://drive.google.com/uc?id=<File ID>)