This feature is only available in the desktop editor.
With the image selected in the desktop editor, click on the Add Image icon
Select type of image to add
You can take a new screenshot, import an image file, or add an image on the clipboard (if an image is on the clipboard).
The image will be added below the bottommost image. With the image selector tool selected, you can select and reposition the image.
Alternatively you can use the contextual menu
Right-click on the image and select an option from the Add Image Using menu.