Side-by-side images aligned on any screen

Question

HTML

<a href="index.html"><img src="imgs/Logo Aguia vet.png" alt="Logo Aguia Informatica" class="responsive"></a>
<a href="http://mactronengenharia.com.br/" target="_blank"><img src="imgs/mactron engenharia.png" alt="Logo Mactron Engenharia" class="responsive"></a>
<a href="#" target="_blank"><img src="imgs/aldosolar.png" alt="Aldo Solar" class="responsive"> </a>

CSS

.responsive { /*Logos rodapé*/
    display: inline-block;
    margin-left: 9em;
    margin-right: auto;
    width: 100%;
    max-width: 250px;
    height: auto;
}   

Answer 1

Your code is almost right, the problem is how you used the class and some properties that are unnecessary...

You put the class right in the image, but the most appropriate to my view would be to create a container and put the class in this container.

.responsive { /*Logos rodapé*/
  display: inline-block;
  /* margin-left: 9em; */
  /* margin-right: auto; */
  width: 100%;
  /* max-width: 250px; */
  height: auto;
  text-align: center;
}   

<div class="responsive">
  <a href="index.html"><img src="https://placecage.com/100/100" alt="Logo Aguia Informatica" ></a>
  <a href="http://mactronengenharia.com.br/" target="_blank"><img src="https://placecage.com/100/100" alt="Logo Mactron Engenharia" ></a>
  <a href="#" target="_blank"><img src="https://placecage.com/100/100" alt="Aldo Solar" > </a>
</div>

Answer 2

If you need images side by side, you can use the display: flex with the properties, justify-content: center; and align-items:center; that they will align all the contents of their div side by side. The display: flex already comes as standard flex-direction: row, if you need the images to be below each other, you just need to change to flex-direction: column.

div {
  display: flex;
  justify-content: center;
  align-items:center;
}

img {
margin: 2.5px;
height: 5em;
width: 5em;
}

<div>
  <img src="https://pbs.twimg.com/profile_images/711520911082569728/CZvvAEVD.jpg">
    <img src="https://pbs.twimg.com/profile_images/711520911082569728/CZvvAEVD.jpg">

  <img src="https://pbs.twimg.com/profile_images/711520911082569728/CZvvAEVD.jpg">

</div>