Place image on top of keywords
<div id="mainContent">some random text. bla bla bla.</div>

var x = document.getElementById('abc').innerHTML;

var keyword = "test";
x.replace(keyword, "<span>"+keyword+"</span>");

document.getElementById('abc').innerHTML = x;

x.replace('b'+ keyword +'b', "<span>"+keyword+"</span>");

var keyword = "random";
var found = $('div:contains("' + keyword +'")');

$.each(found, function(){
    var wrapper = '<span class="whatever">' + keyword + '</span>';
    var oldHtml = $(this).html();

    $(this).html(oldHtml.replace(keyword, wrapper));

});