I don't have the rights to my server to verify this, but I believe the file you're looking for is: $/etc/apps/framework/server/apps/homefx/templates/navbar.html
I'm sure if you uploaded a picture, and replaced the html code to reference the picture it would work... but again, I don't have rights to verify.... and I don't know html coding... I don't know why I answered this, but good luck!
Whether or not that is enough / the right place to start modifying, anything you do down there will not survive an upgrade.