You can add images to the email templates in the Setup Email Content screen

Go to the Campaign tab, click the campaign name and add recipients to the campaign and click the next button. click the Starter templates string and choose the email template from the Setup Email Template screen and click the next button.

In the "Setup Email Content" screen you see the tiles for adding text, images, button, links etc., 

Drag and drop the Image tile to the email content section. The screen then opens and shows an option to browse or uploading the image file, browse the image from the laptop/desktop and add it to the email template.