#Logo{
    Width: 100px;
    Height: 100px;
    Background-Image: URL("Logo.png");
    Background-Repeat: No-Repeat;
    	/*---IE6 Only--*/
	_Background-Image: URL("");
	Filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(src='Logo.png', sizingMethod='scale');
	/*-/-IE6 Only--*/
}

Since there are still many people using IE6, we have to cater for them :-(
Anyway, this CSS statement shows a declaration for a PNG Logo. Most browsers will understand the first 5 lines.  Sadly, IE6 will not be able to use the transparent PNG from line 4 and we override the background declaration by using the "_" at the beginning of line 7 (all other browsers ignore the "_").  Then we use the MS filter in lie 8 to insert the PNG image.

IE6 Only CSS. Also make IE6 use PNG backgrounds

ie6

CSS