As of 31-January-2021, this is how I unlocked it - no dummy accounts or other people needed. Takes about 20-30 minutes, depending on how many pictures you take.
Don't be intimidated by the length of the guide - this is really one of the easiest online achievements you will unlock in these games.
1) Install the Forza Hub app on your Xbox (if you don't already have it, then you're missing out on free credits each week). Go through the setup and make sure it's synced to your account. (Some guides say you can install it on your computer, but there seems to be issues with delays and views not counting when done this way.)
2) Open Forza 5. Start a race. Pause the game and select Photo Mode.
3) Start taking pictures. The more pictures you take, the less time it will take to unlock. Taking 20 means you only have to go through steps 6-11 five times.
4) Save each picture to your Xbox. The second Save box is the Share box - it will prompt you to enter a description, whatever you put doesn't matter. You also don't have to put anything in to help you find your pictures - just save, share and move on to the next one.
5) Once you are done, close out of Forza 5 COMPLETELY (home screen, start, close game fully). It's VITAL that you close out completely each time you exit the game, not just go to the home screen.
6) Open the Forza Hub app. Go to the Gallery tab. Select "My Pictures" in the bottom right.
7) Open the first picture you took (if you took pictures in other Forza games, select Forza 5 in the dropdown box at the top to filter them out). Once it loads, move to the next one. Do this for each picture you took. Each view, REGARDLESS OF WHO VIEWS IT, counts as a "download" here....I don't know why Turn 10 called it a download instead of a view in the achievement.
8) Close out of the Forza Hub app completely (some guides say to keep it open, I closed it just to be safe, I suggest you do the same).
9) Open Forza 5. Go through the intro screens to get to the Main Menu.
10) Check your achievement status. If you took 20 pictures, then you should be 20% of the way through Gone Viral and 40% through New Wallpaper.
11) Close out of Forza 5 completely. Repeat 6-11 until achievement pops.
If it isn't popping or showing any progression, then check your network status - sometimes there's a bit of delay between the Hub and the game.