Part 1 - How to replace default Brio Plone Skin company logo with a new one
Company logo of Brio Plone Skin is displayed at the top left blue area:

You can replace this Brio Skin logo with yours in two ways: through ZMI and through file system.
1. Logo replacement through Zope Management Interface
Go to ZMI interface and select portal_skins in the left navigation menu.
In portal_skins contents select qBrio.

That contains logoBrio.gif image:

This is a default Brio Plone Skin logo image. To set your own logo press Customize button:

Press Browse button to select the your own logo. Click on Upload:

Now you see your new logoBrio.gif image, and it is automatically put into custom folder:

Now when you return to Plone interface, you'll see new logo image (Quintagroup logo):

For the subsequent logo replacements go directly to ZMI portal_skins/custom/logoBrio.gif and upload new logo image here.
2. Logo replacement through File System
qBrio folder has logoBrio.gif image:
Replace this image with your own company logo. Remember that new logo title should be logoBrio.gif as well.
Now restart server so that changes are applied. Now when you get back to Plone interface you will see your new logo at the blue top left area of the site.