Upload to wp-content/plugins and activate.
Visit the WordPress Dashboard Tools Menu:
Click on Rebuild Meta
The plugin runs. You must then rebuild your thumbnails with a suitable tool.
I like Force Regenerate Thumbnails:
http://wordpress.org/plugins/force-regenerate-thumbnails/
Original function by Ross McKay
http://snippets.webaware.com.au/snippets/repair-wordpress-image-meta/