The API is out there but you as the author, designer and programmer need to use your skills to create this.
I disabled the link since it's not required but why duplicate a site? It doesn't make sense to me.
Hello I Visited youtubethumbnaildownload and it's quite good thumbnail downloader i want to develop this type of site how to get API of youtube
Note: Moderator disabled link as not needed to discuss how to find information or write code.