In order to take the most realistic pictures, you have to keep in mind:
-Could the picture be taken in real life? In other words, where will you position the camera?
-Is the lighting ok? Look at pics of real cars and remember them while editing.
-Beware of the many flaws in GT5. Look closely at your pics and you see what I mean.
Cars and smoke don't mix very well in the game, look at the jagged edges.
If you use standard cars, don't get too close or the pixels will stab you in the eye. Yes, quality is that bad sometimes.
-Think about what you want to photograph.
What do you want to achieve with your first pic? I see blurred flying cars and a static, clean background and road.
It's a bit messy like this. Try focussing on the cars and blur the background, you get much more sense of speed.
Also, your cars are all over the place. It's unclear what you want to show; cars? crowd? Circuit?
Choose your subject and make that your centerpiece.
I hope this helps you a bit. I'm not an expert either, just using some basic rules and a decent photoshop programme