Hi
Having placed your video clip onto Video Track 1 and your overlay image on Video track 2, move the red cursor line on the Timeline track so it cuts the overlay image as well as the Video clip and you should now see the overlay. You may or may not see the main video beneath as this depends on the size of the overlay image which may simply obscure it all.
Now click the FX button on the Video Track 2 Image thumbnail, as NADWW suggests. Open your effects list with the green cross and select the Motion effect.
This will place the preview screen in Clip Preview. and you will see the position guides on the preview image.
Note: You must be in Clip Preview to see the View button.
Click the down arrow on the View button and you will (should) have three choices shown in the drop down menu. The middle one is Clip + Background. This is is hown on C-Major's image. Select this option.
Now when you adjust the overlay image in the Clip preview window to set up your initial image position you will see the Main track video clip revealed beneath as the image is moved to one side.
Nat