If you must offer such content then you can hand code you IE6 with AlphaImageLoader declarations. If these images are also links, be sure to add cursor:pointer; to your CSS.
There is none. Your only options to get real transparency working in IE6 are to use a script (like ie7.js or iepngfix.htc) or use filters. Scripts obviously won’t work if JS is disabled, and "filter" is proprietary to IE, so it won’t validate.
The best you can do is use filter and hide it in a conditional comment for IE6. It still won’t validate, but at least only IE6 will get the non-valid code.