Once your image has been uploaded to your Media tab in the Assets pane, it can be used in your page.

When editing a rich text block on your page, you can simply drag and drop the picture from the Media tab into your page content. Simple! Make sure to give your image an appropriate image description.

You could also choose to create a new block and choose the Image block, which will let you browse for an image or you can simply drag the image from the Media tab into the outlined box for the image.