Change Featured Image Size

I’m working on the About page/main page of my WordPress blog, and one of the featured images is displayed at full size, but the three others are displayed truncated, even though all images are the same size. I have no idea how to fix this, or where I would look to do so. As they scroll, one image also seems to fade momentarily in and out, this is the large image.

See what I mean here: https://www.markladner.com

Any advice would be very much appreciated.