For CSS background-images

.yourselector {
       width:200px;
       height:100px;
       background: url(/folder/yourimage.png) no-repeat;
       _background:none;
       _filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(src='/folder/yourimage.png',sizingMethod='crop');
}

Cannot be used with repeating, needs fixed with and height.

For inline HTML images

img, .png {
       position: relative;
       behavior: expression((this.runtimeStyle.behavior="none")&&(this.pngSet?this.pngSet=true:(this.nodeName == "IMG" && this.src.toLowerCase().indexOf('.png')>-1?(this.runtimeStyle.backgroundImage = "none",
       this.runtimeStyle.filter = "progid:DXImageTransform.Microsoft.AlphaImageLoader(src='" + this.src + "', sizingMethod='image')",
       this.src = "images/transparent.gif"):(this.origBg = this.origBg? this.origBg :this.currentStyle.backgroundImage.toString().replace('url("','').replace('")',''),
       this.runtimeStyle.filter = "progid:DXImageTransform.Microsoft.AlphaImageLoader(src='" + this.origBg + "', sizingMethod='crop')",        this.runtimeStyle.backgroundImage = "none")),this.pngSet=true)); }

This requires a 1×1px transparent GIF image