Blur image preview, what's wrong?

Hi, i would like to ask if there is any chance you could help me solve the fact that the previewed image of the gallery is very blur.
You can have an example of what i mean here (in the bottom right corner of the website there is the “humor” section).

https://osservatoriopsicologia.com/

Am i doing something wrong, it is working as intendere or somerthing else? ??

Thank you for your attention and help

https://www.ads-software.com/plugins/nextgen-gallery/