The video below shows the process of change the look of a product image (click the button on the bottom right of the video to expand the view). You can also follow the screen shots and steps below this.
5. Find the correct image size (in the case of product thumbnails the size is 143x93.) 6. Click Re-crop or Tint Image
7. Scroll through the list until you find the thumbnail image you would like to appear. It will be shown exactly how it will appear in the product gallery.
8. Click Choose on the image you like, then Save Changes and your image will become the one you chose.