It is extremely unlikely that you can find a software tool that would allow you to convert mp3 to png image directly. MP3 file format is audio whilst PNG is bitmap graphics format. Nonetheless a lot of users are actually looking for something different, i.e. how to get the embedded picture from mp3 songs (album covers, interpret images etc.).
You will 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.Open MP3 file Open PNG file
A multifunctional media converter for Windows