Featured Image Thumbnail Size

I’m trying to get my thumbnails to be 300×300 on this page: http://mikemasonbooks.com/blog/. Right now, if I inspect the element, it shows that the img is 150×150 in its natural state and stretched to fit the 300×300 dimensions, making the thumbnail all blurry. The one exception is the first entry, which is a png – the rest are jpgs. The original featured image files are all larger than 150×150 so I’m not sure why it’s automatically making it so small. I went into Settings>Media and changed the thumbnail to 300×300 but that hasn’t seemed to do anything.
How do I change this?
Thanks!