Software to batch process cover art in MP3 files?

It has occurred to me that perhaps the reason my car's player sometimes displays cover art and sometimes not is because some of my files have PNG covers while others have JPG covers.

Is there any freeware out there that can batch process all files and subdirectories to convert PNG to JPG? Or perhaps something with a command-line tool so I can construct a batch file myself?

 

This one looks decent. Tried it and it worked.

Bulk Image Converter

 

Thanks, but I was talking about bulk converting cover art images within the MP3 files, not just image files on their own.

 

I think MusicBee can do that.

MusicBee - The Ultimate Music Manager and Player

Or you can try this.

Batch download and embed album cover art