fixing my web slider

my slider features my last 3 posts, but sometimes the photo in my slider doesnt look perfect same in this exemple https://gyazo.com/4b254fe9be67ab0d324b16ecaf64986b

the slider took of the head of the guy in the picture, how can i fix it without modifiying the photo ? cause i know some will tellto resize photo or whatever, if i do that i'll have to do it to every cover photo in the future, i want a once for all solution

With the information you give us here i can say the follow.

1) If the images is in a img tag you can give that img tag a class and give it some css styling.
2) Place your image in a custom div tag and give that div tag a class and in your css give that class a margin and padding of 0 and a width and height of 100%
in this way the image will resized as the parent div.

If you wanna more help please send me the peace of source or pm me with you skype details or something.
I like to help you

What is the dimensions of the slider, have you set it to auto? you could change the image for the slider using media library

Here's a post on some methods to crop photos properly
https://www.binarymoon.co.uk/2014/07/dont-use-timthumb-instead/

This way, every photo you upload will be intelligently resized/cropped to fit so that you won't need to resize the image manually every time.

Hope this helps!