Upload the WP Post Thumbnail plugin to your wp-content/plugins/ directory then activate it within the WP Admin panel under Plugins.

Go to the WP-Post-Thumbnail Settings page in the WP Admin under Settings. Here you will see 2 sections - "Common Settings" and "Custom Thumbnail Settings".   The Common Settings can be left alone unless you wish to change them.  Under Custom Thumbnail Settings, you are able to set the dimensions and custom key name for up to 3 different images.

1) Enter the image dimensions.  Click "Resize"
2) Assign the custom key by entering the Custom Fields name for that specific thumbnail image.  Click "Assign"
3) Click "Save Options"
4 ) Repeat these steps for preset2 & preset3

*Most of our themes use the Custom Fields names thumbnail for the standard post thumbnail images, and featured_thumbnail for the featured content area.   Be sure to check your theme's FAQ page for specific names and dimensions.

After setting up the WP Post Thumbnail plugin it is now time to test to make sure everything is working properly.

1) Open a post edit page and upload an image to the post's media gallery.
2) Scroll down to the WP Post Thumbnail section and click the "Reload" button to refresh the media gallery items.  You should see your image appear as a little thumbnail.
3) Select the image you want to use and crop it to your liking.  Click "Save Thumbnail"
4) Scroll up to the top of your page and click "Update" to update the post.   Please note that the Custom Fields will not populate with the image's Name & URL until you update the post.

Some bugs I have noticed while working with the WP Post Thumbnail can often easily be fixed by deactivating and reactivating the plugin on the Plugins page. If that does not work for you, try deleting and re-uploading the plugin. If you are still unable to successfully get it to work, some other plugins might be conflicting causing it to not work.