discourse/app/assets/stylesheets/common/base/upload.scss

.uploaded-image-preview {
  background: $primary center;
  background-size: cover;
}

.image-uploader.no-repeat {
  .uploaded-image-preview {
    background-repeat: no-repeat;
  }
}

.image-uploader.contain-image {
  .uploaded-image-preview {
    background-size: contain;
  }
}

.hidden-upload-field {
  visibility: hidden;
  position: absolute;
}