discourse/plugins/chat/assets/stylesheets/common/chat-message-images.scss

$max_image_height: 150px;

.chat-message {
  // append selectors to set images to a
  // max height of $max_image_height
  .chat-message-collapser
    .onebox
    img:not(.ytp-thumbnail-image, .onebox-avatar-inline),
  .chat-message-collapser img.onebox,
  .chat-message-collapser .chat-uploads img,
  .chat-message-collapser p img,
  aside.onebox .onebox-body .aspect-image-full-size,
  aside.onebox .onebox-body .aspect-image-full-size img {
    object-fit: contain;
    max-height: $max_image_height;
    max-width: 100%;
    width: unset;
    overflow: hidden;
  }

  .chat-message-collapser
    .chat-message-collapser-header
    + div
    .chat-message-collapser-lazy-video {
    object-fit: contain;
    height: $max_image_height;
    width: calc(#{$max_image_height} / 9 * 16);
  }

  // Prevent overflow of old lazy-yt images
  // TODO: remove in December 2023
  .lazyYT.lazyYT-container {
    border: none;
    a {
      display: flex;
    }
    .ytp-thumbnail-image {
      object-fit: contain;
      height: $max_image_height;
      width: calc(#{$max_image_height} / 9 * 16);
      pointer-events: none;
    }
  }
}