Taking a 200x160 photo image and displaying it at 600x480 would tend to be blurry.
I see two options.
Display the 200x160 at (or at least closer to) its actual dimensions.
Keep the thumbnails for mobile view / lists, but not for any page where it will be an important part of the content.
If you can’t reconfigure / hack / replace / disable that script I don’t think you have much choice but to show the resized images at smaller dimensions.
I’m not convinced you need to write or even touch any code.
I think all you will need to do is not have the thumbnail stuff
( take a large image - make a small thumbnail of it - display the thumbnail large )
applied to “article” pages.
The original image on the left though a slightly different size looks fine to me.