You can't convert mp3 to png image directly. MP3 file format is audio and PNG is a raster image file format. But you are probably looking how to get the embedded picture from mp3 songs (album covers, interpret images etc.).
You need some mp3 tag editor or music management software like e.g. Helium music manager or MediaMonkey. There are plenty of free tools available, designed for editing mp3 tags. With these tools, you can easily edit information about mp3 song, export pictures and texts or other additional info from mp3 file.