In Media Manager:
1) Rename a .jpg image on your hard drive as joomla_logo_black.jpg and upload it using media manager (you will need to delete the old one first)
2) In Global Configuration:
Go to the public side of your site and make sure its offline - you should see the new image your uploaded in the last step.

In this exercise, I created and image to the same dimensions of joomla_logo_black.jpg. I was able to delete the existing file but each time I clicked upload, the original jpg restored itself. I opened the folder and could actually see my new file but could not see my new image on the public side of my site.

I searched Joomla help for an answer to no avail. I am not sure what went wrong here. Are you able to give insight to this?