If you use a Mac you can set it up so your desktop background automatically pulls from a specific folder. Turn this on and set it up to pull from [dropbox]/IFTTT/wallpapers/Desktop.jpg to have it update when there's a new hot post.
If
New hot post in subreddit
This Trigger fires every time a new post becomes one of the ten hottest posts in the subreddit you specify.
Then
Add file from URL
This Action will download a file at a given URL and add it to Dropbox at the path you specify. NOTE: 30 MB file size limit.
Discover more time saving integrations for Reddit and Dropbox