I've actually done at the .htaccess level, as we need to be able to cache static URLs.
Added this to .htaccess...
# Rewrite .gif emoticons for browsers that support APNG (LEGACY; when possible we want to get rid of the gifs, but Edge does not yet support)
RewriteCond %{HTTP_USER_AGENT} (Chrom|Firefox|Safari)
RewriteRule ^(themes/default/images/cns_emoticons/.*\.gif)$ $1.png [L,QSA]
Added this to .htaccess...
# Rewrite .gif emoticons for browsers that support APNG (LEGACY; when possible we want to get rid of the gifs, but Edge does not yet support)
RewriteCond %{HTTP_USER_AGENT} (Chrom|Firefox|Safari)
RewriteRule ^(themes/default/images/cns_emoticons/.*\.gif)$ $1.png [L,QSA]
Now just awaiting Manoj's images.