To Add PNG Light Effect on Photo in Photoshop Easily...

Lets Begin, first Open your Photo on which you
want to add light Effect.

Then, Open your PNG folder and Drag your PNG photo on Photoshop layer.

then, your PNG photo is added in your Photo.

and then, you want some effect on light or decrease Opacity 

See Right-down side and Adjust your Light Effect.

Now Your Pic is Completed... ;) 

Tip : If your Light effect photo in not PNG and in Black Or white Background and You want to add. So add this by drag on layer then go to effects, if your pic's background is white so click on Multiple,Darken,etc & if Your Pic's Backgroungd is Black Choose Screen, Lighten,etc-etc. Thank-you...!