Since we do not have direct option to copy paste images, or to insert images from outside to Notification Templates, then this can be achieved using insert html option in the Editor.
include the image Source as HTML in the Notification Template.
Refer the images below in Notification template,
After inserting the image src in the Edit html box, click on Preview button next to it and check if the image you are trying to insert is in the correct format, and click on save. Now you need to follow the similar procedure for all the notification templates.