MainWP Branding Extension allows you to customize your WordPress (child) sites. With the Branding extension you can easily Change the Login Screen Logo.
This option provides you the ability to replace the default WordPress logo(icon) on the login page of your child sites. To do that, go to: MainWP Branding Extension settings page, locate the WordPress Branding Options section and the first option in the section will enable you to do this:
Click the Choose File button and locate the image you want to use. If you are done with WordPress customization, click the Save Settings button at the bottom of the page.