You can find the Shadow overlay effect in the Toolbox under the Objects tab  in the Live Effects area.

The content of the effect is overlaid by an animatable shadow.

To use the effect, you can  drag it directly onto an image in the Timeline.

You can also drag the effect to the Timeline and later insert one or more images into the effect.

After you insert the image, a colored frame with the title of the effect appears around the small preview image in the Timeline. When you click the colored frame, you can specify the settings for the effect in the Properties area.

In the Properties area  you can now define the settings of the effect.

You can choose from various shadow patterns in the Overlay gallery, including leaves, palm fronds, windows and blind shadows. Select a motif that matches the image material used.

Use the Strength to set the opacity of the overlay pattern. The shadow can also be intensified by activating "Duplicate". This places the overlay graphic twice over the content.

With Reflection, the overlay graphic can be mirrored horizontally and/or vertically and thus positioned to match the image content.
Depending on the value set for the Blur, the overlay graphic is either clearly visible or appears softly blurred.
 
The movement of the shadow overlay can be influenced by the Strength (how far the shadow changes its position) and the Speed (how often the shadow changes its position during the duration of the effect). This can optionally be done with or without rotating the shadow overlay.

If you do not want the effect to be animated, set the Strength and Motion to 0%.

To further animate the effect, the Strength and the Blur can be changed over time using a curve.