func (d *decoder) readImageDescriptor(keepAllFrames bool) error {
	m, err := d.newImageFromDescriptor()
	if err != nil {
		return err
	}
	useLocalColorTable := d.imageFields&fColorTable != 0
	if useLocalColorTable {
		m.Palette, err = d.readColorTable(d.imageFields)
		if err != nil {
			return err
		}
	} else {
		if d.globalColorTable == nil {
			return errors.New("gif: no color table")
		}