I think the most important thing when taking photos is to hide the game's flaws which ruins the realism. When taking a photo of a car, try to experiment with different positions in order to remove or minimalize the ugly shadows.
Also, this game is all about the lightning. Try to find places and tracks with good, neutral lightning. For example, I think it's hard to take a realistic photo on the "normal" Nurburgring because the lightning is a bit flat. But if you go to the time/weather change, you can tweak the lightning yourself for better results. I think that the game has a more realistic vibe to it during dawn/dusk with slightly overcast weather.
Another important point is the reflections (background stuff that gets reflected on your car). Because of hardware limits, reflections are low resolution and that can easily ruin the photo, but it can also make it better if you can find the places and angles where they look good. I would also avoid using filters for taking realistic photos, unless you really know what you're doing.
Of course, for realistic photos you should be taking photos of premium cars, because they are far more detailed and believable (it's possible with standards too, but requires far more attention). Same goes for tracks, the more detailed the track, the easier is to make a believable photo. One thing you should be careful when taking close-up photos of cars is the smoothness of cars. Some cars have blocky edges on curved surfaces, and many people forget that and it ruins their photos.
As far as editing goes, i think editing is a must because some quality is lost when transferring from PS3 to PC, so I would at least sharpen the photo and "restore" the lightning because your TV and monitor are likely calibrated differently, so your results won't be 100% same. For advanced editing, there are guides here on GTPlanet that explain it way better than I could, so check that out. All of this is just my opinion and I'm not an expert by any means but I hope this helps a little bit.
