Replace Kibana logo and loading icon in Kibana 7.0

Have you tried changing it by replacing the file for ex: "abcd.svg" present under the directory "/usr/share/kibana/optimize/bundles/".Images with png extension works as well.
Refreshing the browser alone is sufficient.
Path; /usr/share/kibana/optimize/bundles/src/ui/public/images

More discuss posts with more info

Thanks
Rashmi