What is Unsplash? A website where you can find free images for any topic. Free and royalty-free. Because, believe us on trust, copyright infringement penalties come, so it's really not worth the risk using photos "found" on the net.
Unsplash is integrated in the newsletter editor, so uploading an image is really simple: let's say you are preparing your herbalist's weekly newsletter and you want to put a picture of a lavender field in bloom in the header. Great: just select 'header image' from those available for composing the message, type 'lavender' in the Unsplash search field, choose the one that suits you best, click again to add the image and the game is done.
Once you've uploaded the image you can edit it, add a text, an icon or modify it as you prefer before uploading it inside the message.