<picture class="picture-container">
    <!-- iillimani se muestra a partir de 768px en adelante  -->
    <source media="(width >= 768px)" srcset="https://images.pexels.com/photos/18046333/pexels-photo-18046333/free-photo-of-paisaje-montanas-viaje-bolivia.jpeg?auto=compress&cs=tinysrgb&w=1260&h=750&dpr=1">
    <!-- los flamengos se muetra por defecto en pantallas moviles -->
    <source srcset="https://images.pexels.com/photos/17270283/pexels-photo-17270283/free-photo-of-agua-caminando-aves-laguna.jpeg?auto=compress&cs=tinysrgb&w=1260&h=750&dpr=1">
    <!-- si el navegador no soporta picture se muestra uyuni  -->
    <img src="https://images.pexels.com/photos/2613110/pexels-photo-2613110.jpeg?auto=compress&cs=tinysrgb&w=1260&h=750&dpr=1" alt="uyuni" class="img-defecto">
  </picture>
.picture-container {
  display: flex;
  justify-content: center;
  align-items: center;
  width: 100vw;
  height: 100vh;
}

.picture-container .img-defecto{
  width: 500px;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.