Image fields in content types can be viewed in an image style. Drupal 8 also provides responsive image styles where you can set breakpoint depending on the width of the screen.
Let's have a look.. We shall make a square image style. There for we have to scale and crop the image to maintain the ratio width/height.
Other possibilities. Eg desaturation makes the image black.
The responsive ones are a bit different. You have 2 examples to look at. Narrow and Wide.
You can see the breakpoints of the screen en the size of the image...