Administrators can set a company wide logo that will appear on all quotations created by CQ users in their company. If you don't have access to the Admin settings, you wont be able to do this.
As it can take a few attempts to find the right dimensions for your logo, it may be worth adding it as a default logo on the report viewer and then transferring the dimensions into the admin logo afterwards. Instructions for this are HERE.
To set one, go to the "Admin Settings" section and click on "Company Logo" at the top.
In the pop-up window, click the + and search for your logo (the logo must be a .JPEG or .PNG file) and then use the boxes on the right to set the dimensions and margins for it. Once finished, click the tick button and then "Save Changes" at the top left of the admin settings.
It will now appear on all future quotes sent out by your users unless they have added their own default logo and chosen to override it.