opacity not working for img pointed by Hyperlink

Question

In an asp.net applicaiton I am using Hyperlink which points to an image. On certain condition I am setting its "filter" property to alpha(opacity=70) for IE and "opacity" prop to 0.7 for Firefox so that it is shown dull to depict disable mode. This works fine in FIREFOX ie it shows dull in disabled mode but not in IE.

I see that the filter property is set on anchor tag and not on img tag present inside anchor. If I explicitly put the filter on image it works. But I am not able to do that programatically.

Any idea?