Thumbnail is a small image will show up next to your comments on walls and in search results.
Go to your Facebook profile page at the top right corner of Facebook page.
Hover your mouse over your profile picture.
Click on the “Change Picture” link that appears in the top, right corner of your profile picture.
Click the “Edit thumbnail” link, clickon the image and drag the image to the position you want, click the “Save” icon when finished.