I think that's part of the challenge, recognising the places in the description/picture and working it out from that. There's no indicator on the map to tell you where they are. As @Nicolas mentions, one is located in a q------, which is fairly easy to find if you've explored some of the map / look at the paper map in your game box. The second is almost impossible, third for me was very easy as I'd been there before.
It's one of those things, when you know the answer, looking back at the picture you start to see things which give it away (a special building, the shape of a road, etc) so it's just a case of keep hunting.
One thing I'd say, use the paper map to look up road shapes and the like rather than the in-game map, for the second one I hadn't discovered the area yet on the in-game map so I would never have found it.
Oh, a last thought, if you get the GTA manual (PC install or app I think) it has a very detailed, Google Maps type zoom thing (with aerial view) which you can use to match up like for like what you can see in the picture. If you're really stuck but don't what to cheat I'd suggest trying that.