First, we need to think about what car we want to shrink. If you're not sure, just browse the web for something and you will find it. As for me, I'm going to shrink an Audi R8. Create a new canvas in Gimp and open up your car image on a new layer.
Duplicate the layer. Rename this layer as "mini car." Now use the scale tool and shrink this layer like I have done:
Hide the mini car layer. Now on the original layer, get out the Rectangular Select tool and select the rear wheel, starting from the edge of the tail light, working your way down until the entire rear wheel is selected. Copy/paste the wheel onto a new layer and name it "rear wheel." Hide the original car layer and unhide the mini car layer.
Lower the rear wheel's opacity to about 70%. Position the rear wheel carefully so that wheel fits in the space between the black paint on the door and the rear grill pretty seemlessly. Some parts will overlap, but that's ok, we'll fix it.
Bring the opacity back to 100%. Right click on the wheel's layer and select "Add Layer Mask." Choose the first option and click ok. Use black as the foreground color and select a soft paint brush. Start removing the areas that are not required by painting black on the layer mask. We want to keep the wheel's arch panel, so don't remove that. Use a smaller brush for areas that are harder to get to.
Now after doing this, I noticed how blurry the car was compared to the rear wheel. I did something about this by selecting the mini car layer, and using the Blur Tool on it, making it a little blurry. I also did the same to the rear wheel layer. We will fix the bluriness later.
Now repeate the wheel process for the front wheel. Blur the wheel a little too like we did in step 5.
Looks good eh? Ok now we need to fix the air intake on the bottom of the car. Hide all of the layers except the original car. Use the lasso tool and select the air intake. Copy/paste onto a new layer. Hide the original layer and show the mini car and its wheels. Use the same process we did before when we put the wheels on to add the intake. Blur it as we did with the wheels.
We're almost done. Now we need to sharpen the image a little. Merge all of the layers except the original car, which can be deleted. Name the merged layers "mini car." Resize the canvas to the size of the layer. Go to Filters > Enhance > Unsharp Mask. You'll have to play around with the settings a bit to get a desired result.
Aww isn't it cute? Well there you have it, an easy method to shrink cars. Have fun with it!