<div id="image"></div>

let form = {
  img: "data:image/gif;base64,R0lGODlhEAAOALMAAOazToeHh0tLS/7LZv/0jvb29t/f3//Ub//ge8WSLf/rhf/3kdbW1mxsbP//mf///yH5BAAAAAAALAAAAAAQAA4AAARe8L1Ekyky67QZ1hLnjM5UUde0ECwLJoExKcppV0aCcGCmTIHEIUEqjgaORCMxIC6e0CcguWw6aFjsVMkkIr7g77ZKPJjPZqIyd7sJAgVGoEGv2xsBxqNgYPj/gAwXEQA7", // тут закодированная base64 строка
  name: "smth",
  desc: "smth"
}

let output = JSON.stringify(form);
console.log(output)

var elem = document.createElement("img");
elem.setAttribute("src", form.img);
document.getElementById("image").appendChild(elem);