Edit in JSFiddle

window.onInsert = function() {

  var html = '<img src="http://furodrive.com/wp-content/uploads/2015/03/logo_title.png" style="width: 30px ; height: 30px">';

  document.getElementById("form-text").innerHTML += html;
}

<button onclick="onInsert()">Insert Image</button>

<div id="form-text" contentEditable="true"  style="height: 100px; background: white;"></div>