Read the original full tutorial here,
- type text as usual (black letters on white background), make them bold, enlarge;
- make a screenshot of the text: Command+Shift+4 (⌘+⇧+4), image should appear on your Desktop;
- put screenshot over the photo. It may be easier to open a new blank document and import the photo into it, then the screenshot of the text.
- remove black text with Alpha - you should see your image through letters now.
Here I put a panorama of New York skyline inside the famous I love New York slogan. The heart shape is in the Character Palette>Symbols>Miscellaneous.
Original panorama photo of New York by Daniel Schwen