It would help if you could specify whether your instance is a fullstack or a devstack.
If you are just testing/developing with a devstack, you can change the logo of the header replacing the image in /edx/app/edxapp/edx-platform/lms/static/images/logo.png (PNG, 72x48, 8 bit color)
If you want to replace the broken logo image of the footer, you should change the FOOTER_ORGANIZATION_IMAGE variable. If it is just for testing purposes, you can change if in lms.env.json.
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Greetings,
Juanan Pereira