autoimage.js

var AutoImage = function AutoImage(message) {
  if (_.trim(message.html)) {
    message.html = message.html.split(/ /).map(function (part, index) {
      if (part && part.length && part.length > 0) {
        var codeMatch = part.match(/(?:```(\w*)[\n ]?([\s\S]*?)```+?)|(?:`(?:[^`]+)`)/);
        var imgHrefMatch = part.match(/href=/i);
        if (!codeMatch && !imgHrefMatch) {
          return part.replace(/(https?:\/\/.*\.(?:png|jpeg|jpg|gif))/i, '<img src="$1" />');
        } else {
          return part;
        }
      }
    }).join(' ');
    return message;
  }
};

RocketChat.callbacks.add('renderMessage', AutoImage);

package.js

Package.describe({
  name: 'rocketchat:autoimage',
  version: '0.0.1',
  summary: 'Message pre-processor that will transform image links on messages',
  git: ''
});

Package.onUse(function(api) {
  api.versionsFrom('1.0');
  api.use([
    'rocketchat:lib'
  ]);
  api.addFiles('autoimage.js', ['server','client']);
});