I need to edit an ad and I saw a design that got my attention. I want to add this layer of transparent white to a picture that I have. How would I do that so the end result is similar to the one below?
The example you provided actually has a semi-transparent (dark) grey layer on top of the photo. My guess is at least 50% grey at about 30% transparency. If it were white, the text would be too hard to read. If you make the color darker, you will probably want to make it more transparent. Play around with those two variables until you get the effect you want.
You can add this in either Photoshop or InDesign or Illustrator, whatever you're designing your ad in.