puyopuyotetr.is/app/javascript/mastodon/features/account_gallery/components/media_item.js

import React from 'react';
import ImmutablePropTypes from 'react-immutable-proptypes';
import ImmutablePureComponent from 'react-immutable-pure-component';
import Permalink from '../../../components/permalink';

export default class MediaItem extends ImmutablePureComponent {

  static propTypes = {
    media: ImmutablePropTypes.map.isRequired,
  };

  render () {
    const { media } = this.props;
    const status = media.get('status');
    const focusX = media.getIn(['meta', 'focus', 'x']);
    const focusY = media.getIn(['meta', 'focus', 'y']);
    const x = ((focusX /  2) + .5) * 100;
    const y = ((focusY / -2) + .5) * 100;
    const style = {};

    let content;

    if (media.get('type') === 'gifv') {
      content = <span className='media-gallery__gifv__label'>GIF</span>;
    }

    if (!status.get('sensitive')) {
      style.backgroundImage    = `url(${media.get('preview_url')})`;
      style.backgroundPosition = `${x}% ${y}%`;
    }

    return (
      <div className='account-gallery__item'>
        <Permalink to={`/statuses/${status.get('id')}`} href={status.get('url')} style={style}>
          {content}
        </Permalink>
      </div>
    );
  }

}