Depending on your package you may have the ability to brand your Huddle sign in page, dashboard logo, and individual workspace logos. For overall account customisation, you need to be an account administrator:

In you Account Settings,  you can set up the logos for your custom portal in the customisation section: 

Just choose appropriate files for each section, then remember to save your changes at the bottom. Logon logo appears in when your users are asked for their username and password. Dashboard logo appears in the top right of all your workspaces. Default workspace logo appears in the top left of all new workspaces you create.