You can't. Actually, there are hacks to make it work in IE6 somewhat but you have to google for that.
This may be of some help. http://perishablepress.com/press/2008/05/28/css-hackz-series-png-fix-for-internet-explorer/ PNG's are transparent for IE7 and above and all other browsers.
Actually, you can make transparent .png that works all the way back to IE4 - the problem is it's single color transparency in a palettized image - just like .gif Really I've rarely seen a design where alpha is NECESSARY since most of the time you can pre-composite the image - which has the bonus of smaller filesizes. I avoid alpha .png's not just because of their lack of IE 6/earlier support, but because they are generally a waste of bandwidth.
Please try following: <!--[if lte IE 6]> <style type="text/css"> .class {filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(src='bla.png',sizingMethod='image');} </style> <![endif]--> Code (markup): but you can not use repeat-x for PNG files
Don NOT use PNG images. If using Fireworks check their help for how to create a transparent gif - in bathrooms bury st edmunds I created the "bathrooms designed and installed to a high standard" text as a transparent gif in Fireworks and then overlaid this gif on top of a semi-opaque white background.
I believe you mean don't use ALPHA transparency/true-color .png. Palettized .png has no compatibility problems, and for images of more than 16 colors results in a smaller filesize and is functionally identical to .gif The only real issue is color matching, as IE applies the file gamma to .png's. - if you don't need color matching or have fullbright/dark as 255,0 it's a non-issue, but if you NEED it to work you can fix that by deleting the GAMA line using tweakpng. http://entropymine.com/jason/tweakpng/ Unless you are dealing with 16 color images, we can do a hell of a lot better than .gif since IE added support in version 3. YES, I SAID IE3!!!
Personally I think if it's possible you should try and include the background as part of the image. Obviously this is only possible in certain circumstances, and has the added problem that if you ever want to change certain parts of the design a few images may need updating (not the biggest of deals, depending on the layout).
Since when? It's worked on background images for a while now, even tiles (though it can murder a slow system). Hate having to use alpha png's, then again, me & designers often don't quite see eye to eye when it comes to what should be left to print and what should be included in web design.
Since when? Did they add positioning and tiling support? It might ALMOST be worth using at that point if alpha .png weren't complete bloated trash in the first place usually not worth using except during the design stage before you composite it all down. But then, it seems like people don't seem to be taking issue with having 1 megabyte or larger websites anymore - Morons.
They added that into the new version: http://www.twinhelix.com/css/iepngfix/ Full CSS background positioning and repeat are supporting (including CSS sprites)