Allows you to override WordPress' auto generated thumbnails.
image-override folder to your
Activate the "Image Override" plugin in your WordPress administration interface
Create (or edit) a page or a post with a featured image.
Down below, in the Image Override metabox, upload an alternative image for one of the sizes, and save the post.
Any function you're currently using to display the image (ex: the_post_thumbnail( 'medium' ) ) will now display the new image if the override is applied to that size.