Problem with CSS

Hello.

I have a problem with the theme, the code “max-width: 100%” css that makes images fit (responsive), make some images look blurred (less quality, blurry), here I leave an example .

Image added a widget to the sidebar:

With max-width: 100% (blurred)
http://prntscr.com/5k329g

Without max-width: 100% (original quality)
http://prntscr.com/5k32pq

What line of code should I edit to not pass this?

If I remove that part of the image on mobile is very large, so the solution is not that.

Thank You.