Property src

The src property is used inside image blocks and should hold the location of the image you want to add to your email.

{
    "from" : "info@example.com",
    "subject" : "Email with a single image",
    "content" : {
        "blocks" : [ {
            "type": "image",
            "src": "https://responsiveemail.com/example.png",
            "alt": "Example image"
        } ]
    }
}

For more information and more examples, please check the documentation of image blocks.