To add a logo to your store, first log in to your admin panel. Hover over Design, on the left menu and click Logos.

Here you will be able to choose from three different logo types:
 
 
  • Use Company Name: Use a simple text logo.
  • Upload Your Own Logo: Upload an image or Flash logo
  • Create a Logo: Enter text and choose the font, color, and size (Note: If you choose an uncommon font, it will not display on most people's computers - a font must be installed on a computer in order for it to render it)
You can also upload an "Invoice Logo," which will appear on receipts and packing slips. In addition, you can upload a favicon and mobile logo as well.