How does twitter.com make JPEG profile images transparent?

This image looks like it has a transparent background: https://twitter.com/revifemqueralt/photo

However, inspecting the URL, this is a JPEG file, and so it cannot be transparent (and it is not, it has a white background): https://pbs.twimg.com/profile_images/1419981304121200654/o1zUsgWT_400x400.jpg

Am I missing something?