You want to change this file: /src/ui/views/ui_app.jade
Change the image here
Change the text here
Once you've made your changes, if you are running from source pulled from github, you can start kibana by ./bin/kibana --dev which will watch files for changes and should automatically re-optimize and show you the latest versions.
If you are running from a distribution, you need to delete the files from within the optimize folder for any changes that you made to the src folder to be included.