You can set your company logo in organisation settings for your account, and you can set your client's logo in Step 2 of the company set up. Then, choose which logo is used in the header properties for your report or template.
You can add a logo for both online and print headers in your report. The Report Editor allows for different logos and headers to be used, depending on the delivery method for the report, so it's important to ensure that when editing the header, you've formatted both the "print" and "online" versions accordingly.
Select your logo from the 'more options' icon: