New feature idea: Modify thumbnails, add our own images.

[mhncho]

We can add videos from YouTube as audios to our playlists, liked, but the thumbnail is not in the right format, and the quality of the image is kinda bad. It would be cool if we could modify thumbnails to put one in the right format, since we can already modify titles, authors, lyrics, it would be cooler if we could change these too.