OK, This is to change the default logo that you see on in your Magento PDF Invoices or Magento Invoice Printouts.

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.

Done. And You got it.