Firstly it is better to pick a photo with a neutral background like trees or a wall since the background will be compressed too. A side view is also easier than a 3/4 front view as the angle will not vary so much.
Once you have selected a picture then its best to duplicate that layer just in case you make a mistake.
First thing to do is to select the around the front wheel making sure to get some of the wheel arch and ground in too. I use the Polygonal Lasso Tool to make the selcetion and once its selected, hit EDIT, COPY, EDIT, PASTE. This will paste that selection onto a new layer.
Then do the same for the rear wheel again making sure to include some of the wheelarch and ground/shadow. Once again hit EDIT, COPY, EDIT, PASTE. This will put that selcetion on another layer giving you a total of three layers. You might not be able to see them until you turn off some of the little 'eyes' on the layers menu independantly.
Now make sure you are on the orignal background layer of the vehicle and hit EDIT, FREE TRANSFORM. You will then be able to drag the original pic in by however much you want. I usualy bring it in by about 40%.
Now just move the wheels into where they look roughly correct (in the image I chose they did not line up correctly coz the car is on a 3/4 view)
If you have the same problem as me then you can click on the EDIT, TRANSFORM, DISTORT button and manipulate that layer until it lines up with the body and the shadows.
Once you have them roughly lined up, then slect the eraser tool and drop the opacity (on the top menu bar) down to around 50% and select each wheel layer and feather out the edges to tidy them up. This will make them blend into the original layer. I have turned down the opacity of the original layer in this pic so you can see the feathering.
Once you have finished that then just crop the picture to whatever size you want and you're done! Should take you around ten minutes to do.
If you want to make it look more cartoonish, you can copy things like the dor handle, mirrors or gas tank flap from the original picture and paste them ont the finished product as they will look to big for it now