not required, just makes it easier to get the full resolution image. without the extension will give you https://pbs.twimg.com/media/Dt73AQhWsAg-Yqy.jpg, with the extension will give you https://pbs.twimg.com/media/Dt73AQhWsAg-Yqy.jpg:orig. you can achieve the same thing by just manually editing the url, but this lets you skip that step. also, it depends on the image. if it's small enough, without :orig will be the same as :orig.