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

and at the top area of inner pages:

You can replace this logo with any other 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 qploneskinflow.

That contains logoFlow.gif image:

This is a default Flow 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 logoFlow.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/logoFlow.gif and upload new logo image here.
2. Logo replacement through File System
qploneskinflow folder has logoFlow.gif image:
Replace this image with your own company logo. Remember that new logo title should be logoFlow.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 top area of the site.