Download fig 1 and fig 6 and open them both in Photoshop. This will select just the car image. This will place a copy of the car on the clip board. Click on the sand image to make it active. You should now have one image, with the car on the uppermost layer.
Active layer is shown by paintbrush icon and blue highlight in the layers palette.
Click on the add layer mask icon at the bottom left of the layers palette. This selects only the car. Set your foreground color to black and background color to white in the floating tools palette. Double click the gradient tool to bring up the gradient tool options palette. Drag the gradient tool across the car from bottom to top.