Changing the header Saleforce Logo in the visualforce

Try with this code..

get the logo image element using javascript and change the src value

<apex:page >
    <script>
    document.getElementsByClassName('phHeaderCustomLogo')[0].getElementsByTagName('img')[0].src = 'https://www.google.co.in/images/branding/googlelogo/2x/googlelogo_color_272x92dp.png';
    </script>
</apex:page>

Output

You can achieve this with javascript . This is the example. The header logo is an image with id phHeaderLogoImage . We can change them using javascript and it will be good to go.

Check the below code:

 <apex:page >
      <!-- Begin Default Content REMOVE THIS -->
      <h1>Congratulations</h1>
      This is your new Page
      <!-- End Default Content REMOVE THIS -->
      <script>
          window.onload = function() {
          var element=document.getElementById("phHeaderLogoImage");
        //  alert(element.src);
          element.src="https://pbs.twimg.com/profile_images/657703841085935616/EulXOwHD.png";
    };
      </script>
    </apex:page>