There are four different logo types which come standard with every Magento install. This gives you the flexibility of assigning appropriate logos to the different task. The following list tells you where the logos are located and their use.

• Website Store Logo:
  Used for the frontend of your store.
  It’s located in:
  your_magento_install/ skin/ frontend/ your_skin/ default/ images/ logo.gif
• Email Logo for Email templates:
  For automated emails to your customer’s.
  It’s located in:
  your_magento_install/ skin/ frontend/ your_skin/ default/ images/ logo_email.gif
• Print logo version vs screen website logo version:
  Used when you print a webpage.
  It’s located in:
  your_magento_install/ skin/ frontend/ your_skin/ default/ images/ logo_print.gif
• HTML Invoice and Packing Slip logo:
  For printing html version.
  It’s located in:
  your_magento_install/ media/ sales/ store/ logo_html/ default/
  You can upload or update this image on your backend here:
  System > Sales > Invoice and Packing Slip Design
• PDF Invoice and Packing Slip logo:
  For printing pdf version.
  It’s located in:
  your_magento_install/ media/ sales/ store/ logo/ default/
  You can upload or update this image on your backend here:
  System > Sales > Invoice and Packing Slip Design