i want square thumb (obiouvsly without stretch)
i found some hacks to do this, but the threads are about 1 years old, and i'm afraid to use those hacks with a newer version of k2.

maybe it's a in-core feature now?

I dont know what hacks are you saying, but maybe the best way is that you cut or set the correct aspect ratio to your images before upload with some picture editing software (desktop or online)

i mean those:
getk2.org/community/New-to-K2-Ask-here-first/8923-Crop-to-square-images-for-thumbs
getk2.org/community/New-to-K2-Ask-here-first/134021-creat-thumbnail-by-width-and-height-you-can-input-height-value-on-K2-control-center
getk2.org/community/New-to-K2-Ask-here-first/150871-PLUGIN-Image-Cropping-Fit-WH-and-More

the users who will post articles in my site can't do this, they can't resize images (besides it's a lot of images, a lot of work)
i have to crop images in size 200x200

Why don't use CSS, overflow: hidden and a standard ratio to hide the excess part of the image?

a correct css solution could be background-size: cover,
but i prefer a php resize..so there isn't an option in k2?
could you link a working hack?

i think this option could be very helpful (for next releases..)

We will consider it for future releases.
background size will work only if you change your <img> tags to backgrounds instead.
I think this is an overkill.

As for the links, I am sorry but I have not tested any of these solutions.