I've been trying to do the same frustrating thing for a minute, here's what I've arrived at, had to use different techniques for different browsers.
string injection = "if(navigator.userAgent.indexOf(\"Firefox\") >= 0){;" +
"var headerElement = document.createElement('div');" +
"headerElement.innerHTML = '<img src=\"URLTOSOURCE" style=\"width: 100%; text-align: center\" />';" +
"var body = document.getElementsByTagName('body')[0];" +
"var insertionPoint = body.lastChild;" +
"body.insertBefore(headerElement, insertionPoint);" +
"}else{;" +
"var headerElement = document.createElement('div');" +
"headerElement.innerHTML = '<img src=\"URLTOSOURCE" />';" +
"var body = document.getElementsByTagName('body')[0];" +
"var insertionPoint = body.children[0]; " +
"var unityPlayer = document.getElementById('unityPlayerEmbed');" +
"unityPlayer.parentNode.insertBefore(headerElement, unityPlayer.nextSibling);" +
"var embedTag = unityPlayer.getElementsByTagName('embed');" +
"embedTag[0].setAttribute('style','display:block;width:1200px;height:600px');" +
"};";
Application.ExternalEval(injection);