The first thing you'll want to do is source your desired image on a 3rd party site such as imgbb or imgur.com. Images cannot be uploaded within the portal, and they'll need to be uploaded and sourced elsewhere.

Once your image has been uploaded, right-click on the image and select Copy Image Address.

Head to your Announcements tab in your portal, click on Insert, and select Image.

Paste the copied Image Address in the Source field. Adjust the image size as needed.

Enter any desired text and select the Save button.

You have successfully added an image to your portal. For more information on the Announcements tab, head to this article.