<===> input.scss
@mixin img-opacity($trans) {
          filter : alpha(opacity=($trans * 100));
      -ms-filter : "progid:DXImageTransform.Microsoft.Alpha(Opacity=#{$trans * 100})";
    -moz-opacity : $trans;
  -khtml-opacity : $trans;
         opacity : $trans;
}

img {
  @include img-opacity(.5);
}
<===> output.css
img {
  filter: alpha(opacity=50);
  -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=50)";
  -moz-opacity: 0.5;
  -khtml-opacity: 0.5;
  opacity: 0.5;
}