If you like to replace the logo on your invoices and even in the header of the CRM you can do this simply by clicking on Configurations and Config Logo.
You can use JPG Files and PNG Files (with transparency if you like). The file size is limited to 3 MB and you should use a Square image.
Once the file is uploaded it replaces the Logo with the new File.
When you are using CRM as Cloud please keep in mind, that there is a global Login, so your changes take place after a User of your company is logged into the CRM.