Assuming you're just talking about replacing the logo, your code should work fine and an iFile is an excellent way of accomplishing the image replacement.
if { [HTTP::uri] eq "imagesTHIRDPARTY.png" } {
HTTP::respond 200 content [ifile get MYCOMPANY_logo_img]
}
The image request is part of the total page render and is a sub component of the page. Your users will not see
www.mycompany.com in the browser if you're just replacing the server's response to the image request with your own content. For style purposes, you of course want to make your image the same dimensions as the original.