change Header image logo

i changed the logo of my site but i want to use a diferent logo on the header leaving the index logo just like it is.

Please provide more information or explain more detailed. What is "index logo" and "logo of my site"?

i mean on the login page i got a image logo of my brand, so after login i got the same logo on the header, i want change that header logo for a diferent logo image.

Edit: the logo image rezise all the complete header deforming the web site.