You can either upload your own image or use a stock photo from Unsplash.
Buttondown has a built-in Unsplash integration, allowing you to search & embed high-quality images without leaving the app.
To search for images on Unsplash, click Find image on Unsplash using the slash menu and a modal will pop up. Search for the image that you want and click on it for it to be added to your email.
To upload an image, you can drag and drop the image into the editor, or type /image on a new line to attach the image file.
See a more detailed guide here.