To insert your company logo in the Merchant Central's page header open the Manage Site Domains page by navigating to Manage > Administration > Branding >  Logos & Domains. 

The page shows a list of domains from which you can access the Merchant Central. Each domain can be set up with a different logo. 

More info: Using a Custom Sub-Domain

If you don't have any domains set up on the page, begin by clicking on the Add New Domain & Logo button. Otherwise, click on the Edit button for the domain that you wish to add the logo to:

The Edit Domain & Logo popup now appears where you can configure your domain and logo settings:

Here is a description of the various settings:

• Site Title: Enter a site title that will be displayed in the browser tab (eg. your company name).
• Domain Address: Enter the domain name from which this Merchant Central site will be accessed. This can be your site URL (such as <clientsite>.iriscrm.com) or a custom domain. Use the Test Domain button to make sure the domain is reachable, or click the Open Domain button to open the domain in a new tab.
• Logo: Upload your company logo. The maximum allowed dimensions are 1002x65px. If your logo is narrower than 1002px, make sure that the logo has a white background.
• Favicon: Upload your site favicon image. Allowed image sizes are 16x16, 32x32, and 64x64px.
• Use Primary Group Branding: Select this option if you want to display the user's primary group logo instead of the main logo that you uploaded for this domain. 
• Restrict By Group: Select the groups that can use this domain as the Default Website Address in their group settings. If you select specific groups here, then the remaining groups that you did not select will not see this domain in the Default Website Address dropdown in the group settings. 

When you finish editing your settings, click Submit Changes and refresh the page to see your new logo in the page header.