In short Photopass is at certain rides (Big Thunder Mountain, Pirates, Space Mountain, Buzz, Tower of Terror), the indoor character meets and the Princess lunch & dinner at the Auberge, for sure. The rest is about being lucky. Outdoor characters depend on the weather and photographers rarily roam the park. There are no fixed locations.
For a more elaborate explanation (from DEC '18)
General:
There are three types of photo passes, a temporary one you’ll get from photographers, a 10-day pass for a short visit and a year pass for annual pass holders. If you book your 10-day pass with your Disney hotel stay, you’ll get a discount. Annual pass holders also get a discount (or free with Infinity), and their photo pass will be valid for one year. If you’re not sure yet if you’ll get enough photo’s you can accept the temporary card from the photographer and have other photographers also add the photo’s to that pass.
The temporary pass can only be used in the park. Go to a shop to see your photos and purchase them (or have them transferred to another pass). The other passes have a website and an app where you can download your photos.
Character photo’s will be kept for 14 days and you’ll be able to transfer them from the temporary card to a 10-day pass within those 14 days. You won’t be able to add ride photo’s to a temporary pass, but if you keep track of the photo numbers, you’ll be able to add them to the pass on the same day (exception is Buzz, the photo’s for this ride aren’t kept for more than a few hours).
Characters:
Not all scheduled characters have photo pass photographers. Especially during winter it can be too cold for the photographers to work outside, so they won’t be present. If the program says there will be a photographer, there will be one. But if it doesn’t state it, there might still be one if the conditions are good.
Rides:
There are two ways to add a ride photo to your photo pass. You can add them in the app, there is a scanner or you add the number. Currently, there don’t seem to be QR-codes at the screens with the photos, but there used to be and you could scan them to add the photo. Now you just enter the number on the screen. You’ll need internet to add the photo and it has been reported the photo’s added this way have a lower quality.
The other way to add a ride photo to you photo pass is to go to the desk. The CM will get the photo, center your party and maybe add a graphic over other people (currently only seen on Space Mountain). After this, the CM will scan your card and the photo is added.
Roaming photographers:
There are hardly any roaming photographers in DLP and they don’t do magic shots. They have been experimenting with magic shots during last's summer Lion King festival near Hakuna Matata restaurant.
Occasionally you can find a roaming photographer on the hub or in the Main Street Gazebo, sometimes Place de Remy in the Studios. But they are rare.