Blurry feature image

Since I changed to this theme the old feature image that I initially had in my old blog theme now is showing blurry in the post header. When I add a new feature image they end up looking great, but the problem is that I have a lot of posts that have the old images and they look great in the old theme, and it will be a lot of work to changing them one by one. So is there a way to change this issue? If yes please let me know. I will appreciate it. Best.

The page I need help with: [log in to see the link]