Yeah, the gif export needs some work. But as I mentioned in my other comment, you can set the background color under View > Set background... before exporting.
Thank you for the feedback! Concerning the background: You can set the background color under "View > Set background..." in the main menu to any color and also include a background image. But what I think needs to be done is to inform the user that jpegs don't support transparency or as you said offer a convenient way to set the background color from the export dialog.
