Given how often Valve has to remove inappropriate avatars and artwork from users profiles, they aren't going to give people another thing they will have to moderate. If you really want to make your own backgrounds, make a game and do whatever is required to have point shop items for your game.

That feature already exists! All you need to do is:

• Make a video game
  
• Pay a $100 deposit to get it listed on Steam (you'll get this money back once you sell $1000 worth of copies of your game)
  
• Sell enough copies to real people that your game becomes eligible for profile features
  
• Submit your points shop items and have them reviewed by the Steam Business Team
  
• Click the button to grant yourself a copy of your published points shop items

That's how I uploaded my mini profile background, anyway.