All of the threads up for this were on the 9th or 10th page, so I'll put one up. Don't have any good wallpapers and just came here to mooch off of those with them?
Here's how you can lend a hand, provided you have a steam account.
1- Open up your badges section
2- Click on any section with trading cards you own or with badges you have
3- Click the prettiest card. Any work, but nicer art preferred.
4- A 1920x1080 full art wallpaper of the card will appear, allowing you to open it in a browser and save it
5- Post on thread for others to enjoy

I'll be posting a few of my favorites in here.
Best of luck!