Magento - How to change default product image and logo in magento

- In the admin end we need to go the following menus,

-  System -> Configuration -> Catalog

- You will find “Product Image Placeholders” Looks like you can upload a new placeholder image for base/small/thumbnail. 

- For Logo image,

 - You can change magento logo from back end 

     - system -> configuration -> select your website from left dropdown 
     - And move to design on left navigation  General -> Design
     - You will find “Header” in that 'Logo Image Src' input field you can give image for logo.