How to Remove WordPress Thumbnails

1. Log in to the WordPress dashboard and navigate to 'Posts.' Click on the title of a post you wish to edit.
2. Scroll down on the 'Edit Post' screen and locate a box labeled 'Featured Image.' If you don't see this box, click the 'Screen Options' tab at the top of the page and make sure 'Featured Image' is checked. If you don't see 'Featured Image' under 'Show on screen,' then your WordPress theme doesn't use featured images and therefore uses a different system for creating thumbnails, such as a plugin.

3. Click the 'Remove' link in the 'Featured Image' box to remove the thumbnail. After removing the thumbnail in this way, you can then add a new one by clicking the 'Set featured image' link.
4. Look under 'Custom Fields' to see if your thumbnails are set there. If your theme uses featured images, then you shouldn't have thumbnails set as custom fields. Click the 'Delete' button under any custom field containing a thumbnail.