Ah, its very dude. Open Photoshop go to File > New > Choose Dimensions > Press OK > First, you need to add your render so, File > Open > Choose Your Render > Resize it > Apply Rename it to Render1
Aftethat, File > Open > Choose your wallpaper and drag it down of Render1.
I will make you a fast tutorial
How Can I Put a Backround Behind Of The Render (For HappyViolence)