When creating an invoice, if you choose to use one of the Clear Books Built-in Themes (from the Settings > Configure system > Invoice themes menu), you can add your logo by following the steps in this guide.
Alternatively, you can create your own invoice theme and select the logo when creating the them, please click here to find out more.
Navigate to Settings > Organisation > Details & Tax and click on the Logo link on the left-hand side of the screen.
Click Upload a logo and select the correct file from the computer that needs to be uploaded. Then click Save changes.