Follow the steps in the following link:
It is important to note that when adding an image to your email that you must understand the size of the image to ensure it is displayed correctly in the outgoing email. We advise that some tests be performed prior to sending emails to customers to ensure everything is working as expected.
Given documents is not available in Salesforce Lightning it is advised that the image be placed on the company webserver and referenced from there. Please see the following article for details: