How to "crop" a rectangular image into a square with CSS?

If the image is in a container with a responsive width:

.rect-img-container {
  position: relative;
}

.rect-img-container::after {
  content: "";
  display: block;
  padding-bottom: 100%;
}

.rect-img {
  position: absolute;
  width: 100%;
  height: 100%;
  object-fit: cover;
}

<div class="rect-img-container">
  <img class="rect-img" src="https://picsum.photos/id/0/367/267" alt="">
</div>

(edit: updated from sass to plain css) (edit: Added dummy image for reference)

Assuming they do not have to be in IMG tags...

HTML:

<div class="thumb1">
</div>

CSS:

.thumb1 { 
  background: url(blah.jpg) 50% 50% no-repeat; /* 50% 50% centers image in div */
  width: 250px;
  height: 250px;
}

.thumb1:hover { YOUR HOVER STYLES HERE }

EDIT: If the div needs to link somewhere just adjust HTML and Styles like so:

HTML:

<div class="thumb1">
<a href="#">Link</a>
</div>

CSS:

.thumb1 { 
  background: url(blah.jpg) 50% 50% no-repeat; /* 50% 50% centers image in div */
  width: 250px;
  height: 250px;
}

.thumb1 a {
  display: block;
  width: 250px;
  height: 250px;
}

.thumb1 a:hover { YOUR HOVER STYLES HERE }

Note this could also be modified to be responsive, for example % widths and heights etc.

1. Place your image in a div.
2. Give your div explicit square dimensions.
3. Set the CSS overflow property on the div to hidden (overflow:hidden).
4. Put your imagine inside the div.
5. Profit.

For example:

<div style="width:200px;height:200px;overflow:hidden">
    <img src="foo.png" />
</div>

A pure CSS solution with no wrapper div or other useless code:

img {
  object-fit: cover;
  width: 230px;
  height: 230px;
}