<div class="triangle"></div>
<img src="https://picsum.photos/id/1/200/300" class="triangle">

.triangle {
  width: 200px;
  aspect-ratio: 1;
  clip-path: polygon(0 0,100% 100%,0 100%);
  background: linear-gradient(90deg,red,lightblue);
}
img {
  object-fit: cover;
}

body {
  display: flex;
  gap: 10px;
}