Thumbnail is a small image will show up next to your comments on walls and in search results.
Step 1
Go to your Facebook profile page at the top right corner of Facebook page.
Step 2
Hover your mouse over your profile picture.
Step 3
Click on the “Change Picture” link that appears in the top, right corner of your profile picture.
Step 4
Click the “Edit thumbnail” link, clickon the image and drag the image to the position you want, click the “Save” icon when finished.