Hummingbird's report is giving the following information:
Optimize the following images to reduce their size by 134.8KiB (55% reduction).
Compressing https://yoursite.com/wp-content/uploads/2016/03/avatar-webinar-bannerCustomer-square.jpg could save 54.2KiB (73% reduction).
(That's just one of the images it's reporting).
This image, along with the others hosted on my server have already Super-SMUSHED, so I'm not sure why it's picking these up.
I've given support access to my dashboard for this.