There are 3 ways to add images to a page within a Beacon document:

You can add an Image Card to the page from the drag and drop menu on the left hand side:

If you want to add multiple images in a gallery format you can drag the Gallery Card onto the page:

See this article for more information on using gallery cards 

If you have a Text Card on a page you can add an image via the formatting menu. Just click the relevant Text Card and then select the 'Images' button as shown below:

See this article for more information about managing images in a Text Card.