I simply discovered that by transforming PNG32 to PNG8 via Illustrator will fix the PNG transparency bug in IE<=6.

And So I had this believed that rather than serving PNG32 to any or all browser, why don't you serve PNG8 when the client is applying IE<=6.

I am no expert if this involves htaccess/httpd directives so I am for help.

The title may be the psuedocode itself.

I've not really attempted this, but It should work:

RewriteEngine on
RewriteCond %{HTTP_USER_AGENT} ^Mozilla/4.0\ \(compatible;\ MSIE\ [1-6]\.
RewriteCond %{REQUEST_FILENAME} ^(.+)(\.png)$
RewriteCond %18%2 -f
RewriteRule ^(.+)\.png$ $18.png [L,QSA]

The very first line turns mod_rewrite on (and may be overlooked if you have done that). The 2nd does the IE <= 6 filter. The 3rd is principally to separate the filename for that 4th line, which inspections to ascertain if <filename>8.png is available. The 5th line really does the redirect.

Put this inside your .htaccess

RewriteEngine on
RewriteRule ^/(.*)\.png$ /$18.png [L,QSA]

Should work.