Myself and others always upload our pictures to the same Dropbox folder. Nis when everr a new picture is uploaded it will be set to your wallpaper
If
New photo in your folder
This Trigger fires every time a photo is saved in the folder you specify. NOTE: will process up to 15 photos per polling period.
Then
Update device wallpaper
This Action will update the wallpaper on your Android device from the image URL you specify.
Explore more great ways to automate Dropbox and Android Device