You can change the company logo that prints on the purchase order through the admin console.
Steps:
1. Login as Admin
2. Then go to Configurations -> Business units.
3. Under Business select the business unit for which you want to change the log and click on Edit.
4. Click on choose file to select the new logo and then click on "Update Business Unit"
Note: Please make sure that the image is 150x150 px or lower.