Why is it converting album art to png?

When I copy an image to the album art pane, and save it to metadata, it always ends up storing as png format even if the source image is jpg. The image in the tag needs to be jpg for my mp3 player to display it. If I select the image from the hard drive rather than drag and drop (copy/paste) it seems to retain the original image format. Is this something to do with Songbird or is it something to do with the OS's clipboard (I'm running Ubuntu)?