A basic example of using FIddlerScript to substitute HTML tags.
if (oSession.HostnameIs("www.example.com") && oSession.oResponse.headers.ExistsAndContains("Content-Type","text/html")){
oSession.utilDecodeResponse();
oSession.utilReplaceInResponse('<b>','<u>');
}
Using the above methods, you can find the tag responsible for the logo creation (probably a <img> tag) and replace it with your own version pointing to another photo.