Just extract its content (should be an image and its config file named the same) anywhere you want.
Then in Retroarch, once you loaded the game you want to have that overlay shown, go to the Retroarch quick menu with F1, then press the back button to get to the main menu, scroll to the left to the Settings and surf down until you reach the Onscreen Overlay option.
There you look for the cfg file in the folder you extracted.
Remember to set the opacity to 1.0 and if the game screen goes besides the overlay, to go in the Video section and play either with the position and size of the screen, or with the integer sacale.
Happy tweaking!