Prova così:

<DIV ID="AlphaId" STYLE="width:X; height:Y; filter:Alpha(Opacity=50, FinishOpacity=100, Style=0, StartX=0, StartY=0, FinishX=320, FinishY=240)">
[img]image.jpg[/img]
</DIV>

(ovviamente da adattare alle tue esigenze...)