1. Login to Instagram using Google Chrome.
2. Go to View menu -> Developer -> Developer Tools.
3. Click the Phone icon to mimic a mobile device.
4. In some cases, you have to reload the Instagram Page in Chrome.
5. Now, you should see the "+" camera button at the bottom of the screen, just like the app. Click it.
6. Find the video or picture you want to upload. Remember it has to be a .JPG for picture or .MP4 for video file. Follow the prompts in the browser window like you would in the app and post!