var image = document.querySelector('img');       // Image you want to save
var saveImg = document.createElement('a');       // New link we use to save it with
saveImg.href = image.src                         // Assign image src to our link target
saveImg.download = "imagename.jpg";              // set filename for download
saveImg.innerHTML = "Click to save image";       // Set link text
document.body.appendChild(saveImg);              // Add link to page