You’re just need to reverse the tags in the jQuery. And instead of href get src of image.

$("img").each(function() {
  $(this).replaceWith("<a href='" + $(this).attr("src") + "'><img src='" + $(this).attr("src") + "'></a>")
})

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.7.1/jquery.min.js"></script>

<img src="https://placehold.co/200" alt="image1" />
<img src="https://placehold.co/200" alt="image2" />

Solution :
I had already done a similar action for a website, you can modify your code like this.

$("img").each(function(index) {
    var imgSrc = $(this).attr("src");
    var altText = "image" + (index + 1); 

    $(this).replaceWith(
      $("<a>")
        .attr("href", imgSrc)
        .append(
          $("<img>")
            .attr("src", imgSrc)
            .attr("alt", altText)
        )
    );
});

This code will allow you to modify all the images that it detects in the code.

Results :
You should get the results as requested.

From

<img src="https://images4.imagebam.com/qwert.jpg" alt="" />
<img src="https://images4.imagebam.com/werty.jpg" alt="" />

to

<a href="https://images4.imagebam.com/qwert.jpg"><img src="https://images4.imagebam.com/qwert.jpg" alt="image1" /></a>
<a href="https://images4.imagebam.com/werty.jpg"><img src="https://images4.imagebam.com/werty.jpg" alt="image2" /></a>