:: before is not displayed in img

I created a sort of lazyload (preloading images) in JavaScript with the scroll event where some images only load when they enter the visible area of the window.

For this I created a generic image of 1x1 transparent pixel ( dt.png ) and placed in the src of the images and a data-src attribute with the path of the respective image:

<img src="dt.png" data-src="img/foto1.jpg" class="lazyload">
<img src="dt.png" data-src="img/foto2.jpg" class="lazyload">

When doing scroll , the JavaScript code will check if the image is inside the viewport window, if it will it will replace the value of src with data-src loading and displaying the image . So far so good.

I tried to do this using the ::before pseudo-class in the <img> tag and even works if the src is empty:

.lazyload::before{
   content: 'Carregando imagem...';
   position: absolute;
   right: 3px;
   top: 3px;
}

div{
   width: 300px;
   background: red;
   padding: 15px;
   color: white;
   text-align: center;
}

img{
   position: relative;
   width: 300px;
   height: 200px;
}

<div>
   <img src="" data-src="img/foto1.jpg" class="lazyload">
   <p>Texto qualquer</p>
</div>

  

But with src empty is not interesting because it shows the "   image start "as well as a border.

Now with transparent PNG, the text of ::before does not appear (even if the PNG is transparent):

.lazyload::before{
   content: 'Carregando imagem...';
   position: absolute;
   right: 3px;
   top: 3px;
}

div{
   width: 300px;
   background: red;
   padding: 15px;
   color: white;
   text-align: center;
}

img{
   position: relative;
   width: 300px;
   height: 200px;
}

<div>
   <img src="https://i.stack.imgur.com/Qk53m.png"data-src="img/foto1.jpg" class="lazyload">
   <p>Tem uma imagem transparante aqui em cima</p>
</div>

  

Looking at the element inspection of the browser, I noticed that when the    src is empty or with an image path that does not exist, the    ::before appears, but when I put a correct image path, the    ::before some.

Is there any way to make this ::before visible even with transparent PNG in src or another possible solution for this case using CSS?