Just run this portable tool and click on Load Picture to add a PNG image to replace the default Windows 7 Logo. After that, click Apply to replace the branding logo. It must be noted that the provided image should be roughly 361 x 58 in size, in order to avoid getting a stretched and de-shaped look for the branding logo.
This will replace the Windows 7 branding logo on your logon screen. In case you would like to revert to the default logo, launch Windows 7 Logon Branding Changer and click Restore.