The Vanishing Point Filter was first introduced with Photoshop CS2 so you will need Photoshop CS2 or later for this project.
The Sacré Coeur image is mine and the computer screen is © iStockPhoto, Dmitry Kutlayev.
Size the image to place on the computer screen to the approximate size the image needs to be having regard to the size of the computer screen image. The computer screen I used is 1600 x 1200 so I sized my photo smaller than this.
Create a new blank layer on the computer screen image (Layer > New > Layer) and then switch to the image you want to add to the screen or billboard. Choose Select > All and then Edit > Copy. Return to the computer screen image.
If the grid is red, it is not a grid that can be used so you need to readjust the corners until it turns blue. You can adjust the grid size if that makes it easier to see.
Drag the image over the grid and you will notice that it reshapes to conform to the perspective of the grid.
To finish, you can add a curves adjustment layer that lightens the image to match the screen lightness. You can also add a gradient to the curves layer mask to adjust the lightening of the image to match the original screen lights and darks.
I also added a shadow to the layer by selecting the layer and select the Add a Layer Style button at the foot of the layer palette. Select Inner Shadow and create an inner shadow for the image so it appears more realistically situated on the screen.
Using a similar technique, you can place a photo on an image of a billboard or any other similar flat but angled surface.