You cannot save alpha channels with the image. The GIF89a Export command lets you convert an RGB or indexed-color image to GIF89a format, specify interlacing, and define background transparency. The GIF89a format supports single alpha channels. The PNG format supports grayscale and RGB color modes with a single alpha channel, and Bitmap and indexed-color modes without alpha channels. When saving in PNG format, you can select Adam7 to display the image in increasing detail as it is downloaded.
Sub optimizes the compression of images with even horizontal patterns or blends.
Up optimizes the compression of images with even vertical patterns. Average optimizes the compression of low-level noise by averaging the color values of adjacent pixels. Paeth optimizes the compression of low-level noise by reassigning adjacent color values. Adaptive applies the filter Sub, Up, Average, or Paeth best-suited for the image. Select Adaptive if you are unsure of which filter to use.