Make sure that your host has the GD graphics extension for PHP enabled.
Upload the plugin file to your wp-plugins/ folder.
Go the plugin management page and activate the plugin.
Add the p75GetThumbnail(int $postID, [int $width, [int $height, [string $fileType]]]) function to your theme to get the URL of a thumbnail and p75GetOriginalImage(int $postID) to get the URL to the original image.
Go to Settings > Thumbnail Options to configure default values for the plugin.