I have a bug with animation of Png Transparent on IE7 (and IE8beta)

with this code :

//mydiv contain a img tag with src=png (an arrow with transparency)

mydiv.slideIn('l', {
                    duration: 0.5,
                    concurrent: mydiv.fadeIn({
                        duration: 0.4
on Firefox, it's working since 1.5 up to 3.0 (of course )
on IE6, it's working with iepngfix.htc

but on IE7 (and IE8 beta), an black outline appears until the div stop the animation
(on chrome beta, idem, the outline is white)

when I stop the fadeIn, the animation works fine.

can you help me?
have I to stop the fadeIn for IE7, IE8, and chrome?