Here's a moderately easy photoshop technique to do: Changing wheels.
This is a lot easier, if you can find the wheels you want to shop onto your car on another car, and hopefully at the same angle. It gets a little harder, and a little less realistic if you have just a picture of the alloy, and have to add it, but it's still possible.
For this example, I'll be using a set of alloys already on another car at roughly the same angle.
I shall be taking the wheels from the car in front, and putting them onto the one behind. Firstly, I'll flip the image, so that both cars are facing the correct way, to get the callipers right etc.
Once I've done that, I'll carefully select the wheel by using the magic wand tool [You can also use the lasso tool, if you feel more comfortable with that though].
Tip: What I find easiest when selecting a wheel, is to select a bit of the tyre too, so you can bledn it into the existing wheel, so it won't look like the new wheel's just been stuck on.
Once the wheel's been selected, use the copy, and paste to past it onto the other image where you want the wheels on. Then use the Edit > Transform > Scale to make the tyre the right size, and also you can use distort to scale the image to the correct size.
Once you've done one wheel, do the same for the other one too.
Tip: Sometimes, if the pictures are of different quality/light source, you may need to adjust the brightness/contrast, which I had to in this case, as the actual wheel was too dark, compared to my picture.
Once you're done, it's time to save, and marvel at your photoshop skills!