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Upload the image-override folder to your /wp-content/plugins/ directory

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.