The PNG and JPEG file formats.

Hi: I've just found a photo file sharing site to upload some images. I had installed, from slackbuilds, nted, a musical score editor. Was very happy with it and, wanting to make a try, I wrote some little music piece, nted produced the object file from my source, and then, to upload it to the mentioned site, I had only to transform the object into some of the formats nted offers. Among these is PNG. I converted to PNG and, when trying to upload to the site, I read the following message: "We only accept files in the JPEG format". Is not PNG a popular format for images? And could I convert PNG files to JPEG?