There appears to exist no way how random JPG images could be converted to MP3 audio. These two file types are of completely different purpose and cannot be converted, because they contain different kind of data.
However, sometimes users want to import .jpg thumbnails as preview pictures when the song is played in a media player. This is typically not possible for MP3 format and the pictures are saved as separate files that share the file name of the song. Only more recent audio formats offer this function and allow pictures to be stored within the multimedia file.
Updated: January 17, 2017