poor image quality post cover

Hello, firstly thanks for the template, I really like it.
My question is: Why, when I upload a cover for a post, does it become of very poor quality? I tried to load and fullhd images and 2k and 4k total is the same, the image as if converted by the theme into quality is worse. Please tell me where it can be corrected? Thank!

p.s. I am from Russia and I do not understand English well; Google used a translator for this message.
The original size of this image is 3840×2160