Suppose you have uploaded an image, and placed it on your page or post as a thumbnail, but you’ve found you don’t like the part of the image which WordPress has used to make the default 150×150 pixel thumbnail. Here is how to change the thumbnail:
STEP 1: Sign in to your site dashboard and click Media to show the Media Library
STEP 2: Find the image you want to change, mouse over it, and click ‘Edit’
STEP 3: Click the ‘Edit Image’ button
STEP 4: IMPORTANT – select the Thumbnail radio button (near the middle of page)
STEP 5: click and drag in the image area to define a new region – best to make it a square area, as the thumbnails are by square default. You can make sure it is square by typing numbers in the Selection boxes near the middle of the screen.
STEP 7: Click the SAVE button under the image
STEP 8: Click the UPDATE MEDIA button
STEP 9: repeat steps 2-8 if you wish to do other images at the same time.
STEP 10: go back to the page with the image thumbnail, delete the thumbnail you placed there previously, and add a new image – remember to use the image from the Media Library Tab, don’t re-upload it from your computer (if you do, you’ll be back to where you started!)