Fill Color of PIL Cropping/Thumbnailing

I am taking an image file and thumbnailing and cropping it with the following PIL code:

        image = Image.open(filename)
        image.thumbnail(size, Image.ANTIALIAS)
        image_size = image.size
        thumb = image.crop( (0, 0, size[0], size[1]) )
        offset_x = max( (size[0] - image_size[0]) / 2, 0 )
        offset_y = max( (size[1] - image_size[1]) / 2, 0 )
        thumb = ImageChops.offset(thumb, offset_x, offset_y)                
        thumb.convert('RGBA').save(filename, 'JPEG')

This works great, except when the image isn't the same aspect ratio, the difference is filled in with a black color (or maybe an alpha channel?). I'm ok with the filling, I'd just like to be able to select the fill color -- or better yet an alpha channel.

Output example:

How can I specify the fill color?