They are really all over. Once you start keeping an eye out, and maybe asking CMs, you'll start noticing them more.
DL ...There are usually people in front of, and towards the side of, the castle coming from Main Street. That's a good start. The castle photographers can do the special picture where they put Tinkerbell in your hand.
During the holidays they hang out where the big pumpkins and later where the big tree stand, so you might find them around that area, if you look around.
If you go to find Jack Sparrow on Pirate's Lair, he's got a whole set-up on the island. When he's there, in my experience, photopass is there. See the end of my trip reports for the Jack Sparrow pix. Actually, see the end of my trip reports for ideas on where to look for photopass people!
In Critter Country they stick around the characters that hang out over there.
In December, there was a photopass guy with Mickey in mickey's house in ToonTown.
I guess maybe other than the castle we usually see them with characters, but someone who was there in September right before me found a photopass person who was willing to walk with them for a little while and showed them all sorts of fun picture places, and took their picture, too. I keep wanting to do that, but we're always in such a hurry to get from point A to B!
In DCA we have always seen a bundle of photopass people right inside the gates, then further along (you can have your pic taken on either side of the golden gate bridge), then over in the big meeting area past Greetings from California (it has a name, I can't remember it).
I have never NOT seen one on the street towards Tower of Terror. That photog (and those near the plaza area) can take the "Stitch" picture, where you point at the ground and they put Stitch popping out of the ground into the pic in their camera. Though if you look at my reports you know that the photog didn't manage to do it, but I contacted photopass and they did it for me after the fact.
We've also found them on the road past GRR, but on the other side of it, so your picture is in front of the mountain.
They have also been at the Mater and Lighting McQueen meeting area near Soarin', though once they weren't there BUT there were CM who were happy to help with a picture anyway.
On the bridge with CA Screaming in the background we've seen photopass people taking the pictures that "put" the Mickey ears of the coaster's loop "onto" your head. silly picture.
I hope that helps a bit.
For what it's worth, if you prepurchase the CD but then don't get enough pictures to justify it to yourself, you can let them know and get a refund. On the other hand, if you're buying the one that allows editing (the prepurchase or the one after you get home) each one of those pictures can become 2 or 3 or 6....each time you crop and/or edit (add borders, add pixie dust, etc) it saves it as a copy, so 10 pictures can easily become 30 or more if you do enough with them. You can add signatures to appropriate character photos, too.
And I like to look at it as buying the rights to the pictures. If you want some pictures printed right then, look at the price of those prints vs the price of having them on CD to print on your own in perpetuity. if you just get the one print and you want a copy, you'll have to find a photo place that doesn't mind you copying that. Some people have a hard time finding such a photo place. But if you were going to order several pictures it would probably be better to think about the CD, because you can make as many prints as you want on your own time.
) Oh, and I think some of the character photos were included on there as well. 
They were able to add it in for me once I got specific enough. Hope that helps; if not you, perhaps someone else!
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