To use phpMyAdmin follow the steps below:
Login to phpMyAdmin panel and select your WordPress database.
Click on the SQL tab which will bring you to a page with a SQL query box.
Once you see the SQL query box, like the image shown below, you can run your SQL query there.
UPDATE wp_posts SET post_content = REPLACE (post_content, '<p style="text-align: center;"><img src="http://i.imgur.com/picture.jpg" alt="" /></p>', '');