If you view the source for the web page, you would see .gif files were
being presented for the images, not .png files.

Web servers can see what web browser is connect to them. They can
present different web pages based on the web browser used. Javascript
can also detect which web browser is rendering a web page. It can
change the web page so its code matches the abilities of the web
browser. So that .png is seen for Firefox and .gif seen for Internet
Explorer means the server or Javascript decided to use a different file
to best match the abilities of the web browser being used.

PNG was not supported in all versions of web browsers. Some web sites
or pages will detect the version of the web browser and decide if it
supports PNG format. Some just want backward compatibility with as many
versions of a web browser as possible so they use the least common
denominator approach by using something that all versions of the web
browser can support.