ResizeFly

Often your NGINX is configured in a way to serve the image as a static resource and in case of failure throw a 404 Not Found error.

To work around this, make sure the request is sent to WordPress when the image cannot be found.

Add a directive like:

location ~* (/[^/]+/)?uploads/(.+\.(png|gif|jpe?g)) {
    try_files $uri $uri/ /index.php?q=$uri&$args;
    expires max;
    log_not_found off;
    access_log off;
}

If you have one long location directive listing all the static file formats (css, js, zip, etc.), you can also just drop the try_files $uri $uri/ /index.php?q=$uri&$args; in there.

The plugin stores an optimized duplicate of each image in this folder. The reason for this is, that image resizing puts a strain on your server and uses comparably a lot of resources. To minimize this ResizeFly creates an optimized copy from which the smaller image sizes will be created.