// Add the code below your theme's style.css if you need to change all employee images
div.emd-container .person-img {
    border-radius: 0;
}

//If you want to change it for shortcode bases, wrap your shortcode in .square class

<div class="square">[employee_circle_grid filter="tax::departments::is::it;misc::posts_per_page::is::-1;"]</div>

// And add the following code to your theme's style.css file

.square div.emd-container .person-img {
    border-radius: 0;
}