First your need to select which images you want to add and find their Image ID.
To find the Image ID, Go to ‘Media’ then ‘Library’:
Next you need to copy that image ID, and Paste it in the editor under ‘Random Images’ then ‘Add Images’:
You then add the images by pasting the Image ID into the row of numbers that looks like this:
Be sure to avoid making any changes to the surrounding text, as this is what defines how the gallery looks.