1. Login as the Management User
2. Click “Site Setup” at the top left of the website
3. Click “Zope Management Interface” near the bottom of the list of links
4. Scroll down and click “portal_skins (Controls skin …)”
5. Click “plinkit_custom” (If you are using one of the themes that has a colored logo, you may need to go into that folder 'skin_blue', 'skin_grey', etc to find the logo you want to change)
6. Click “mylogo” or "banner"
7. Click the “Browse…” Button
8. Select your new logo/banner and click “Open”
9. Click “Upload”
10. Click “Save Changes”

You may have to refresh the page two or three times, but it should now display your new logo.  If refreshing doesn't work, then try to hit Ctrl-F5, that will do a hard refresh in IE.  If it still doesn't show up, then you may have loaded your logo into the wrong area.  Be sure to drill down into the specific folder in "portal_skins" that applies to your skin name.