Optimizing images with Hummingbird

Hello!

Many pictures do not get optimized due to an error. As a result, I receive a very low score on image optimization. All these pictures are .PNGs used for buttons and created by our designers in PS.

Any ideas why Hummingbird cannot optimize them? And what would be the optimal solution in this case?

Here is also the screenshot to the report in the dashboard: http://prntscr.com/emnzxs

And the actual links:
https://www.studentsforliberty.org/wp-content/uploads/2015/01/buttons_ReadOurHover.png

https://www.studentsforliberty.org/wp-content/uploads/2017/02/COVER_01-960x750.png

Thank you.