How to change default Logo for Magento Magento PDF Invoices & Print Outs
– In your Admin Console. Go to Configuration -> Sales -> Invoice and Packing Slip Design
– Delete the Logo by checking the Delete Image box, or simple upload your own logo. It seems that the logo extension has to be *.jpg. And your logo should be 200 x 50 px, otherwise the pdf generator would scale whatever image you give it to 200 x 50 px.
– Save Config.