You will want to explore the Media Settings page in the Admin panels of WordPress. wp-admin/options-media.php
Crop thumbnails is the checkbox that cuts off pictures.
The sizes you put in the other fields determine how each picture is resized during the Upload process. In fact, there can be four or more copies of the picture, as resizings are done, if necessary, for the Thumbnail, Medium and Large (there is also Full Size, which is the original picture).
Not knowing whether your web site uses the Thumbnail, Medium or Large resizing, I cannot tell you which of the Sizes needs changing in that panel.