1) dont use ie8 with option "work as ie7". it lies in this case 2) use clean ie7 or ietester last version
For example test.png can be 50% tranparency jpeg picture. I know 2 methods to use it in ie7:
background-image: "test.png"; filter: alpha(opacity=70)
you will see gray image
filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(src="test.png")
alpha(opacity=70);
background: none;
you will see white image
2 variant came to us from ie6!
example: http://pastehtml.com/view/awfp66p2m.html 1 original. 2 grey background. 3 white progid
ie7 screenshot: http://clip2net.com/clip/m46295/1307529587-cliptj2620-2kb.png
ie6 screenshot: http://clip2net.com/clip/m46295/1307529630-cliptt2620-2kb.png
3 image works perfectly in ie6 and ie8. it means in ie6 and ie8 i can animate opacity using jquery!