Are these repeating, background, content using img tags, or something else?

The png fix module works but I am now a fan of doing it via CSS when ever possible. It's pretty simple, actually... for the cases you can use it.

in this case, it would be filefield plus imagecache uploads of PNG images.

What is the CSS method you use to get PNG transparency in IE6?

In an IE6 specific stylesheet put this in...

.myThing {
  background: none; 
  filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(src='/path/to/image.png', sizingMethod='crop');
}

You'll need to do this for each image and it doesn't to repeating images. You'll, also, need the full patch from the base of the site.

Filter is a specific MS thing. This invokes a feature not enabled by default which can handle transparencies.