When you you start up SmartPlaylist you can tap "⌥" (Options) and select "➜ Now Playing" ...
... to jump directly to the song which is now playing on the iPhone:
Here, you simply double-tap on the artwork to enlarge it to full screen:
With one finger you can move the artwork left or right until you get the desired frame. Now you press the home button and the on/off button on top at the same time to take a picture of what you see on the screen (you will get a white flash when this happens).
Then you go to the iPhone settings and select "Brightness & Wallpaper". You will find your full screen artwork picture in your Camera Roll.
So, with just a few taps you get a wallpaper from the artwork in your music library.