Finally, I found the way to customize facebook wall post thumbnail for fanpage. After uploading a profile picture in Facebook (or facebook fan page), by default random thumbnail of your picture will be displayed for all your Wall Posts. I thought, it is not possible to pick an area within the picture to be used for thumbnail, but I was wrong.
Steps to select thumbnail from profile picture for facebook wall post.
- Upload the image for facebook profile picture.
- Click on the Image (after uploading). It will open Profile Pictures Folder.
- Click on Change Profile Picture Button on Top Right side.
- Mouse Cursor will change if you move it Thumbnail Version.
- Select the desired area within the picture for wall post thumbnail.
- Save Thumbnail Version.
This is useful when you like to display just the part of your logo (instead of some random area).