Position Editing
The preview is where you spatially arrange media in your project.
Clicking on any object in the window will select it, and you can then resize it, or move it around.
You can use the handles on the edge of the media to resize the image.
You can use the rotation handle 
at the middle of your media to rotate around the Z
axis (tilt the image).
To rotate around the Y or X axis (in and out of the screen), please see the
Video Properties.
Here the screencast has been resized:
Special keys while editing
SHIFT—when resizing the media aspect ratio is respected.
COMMAND—snapping and guides are ignored.
CONTROL—cropping is performed on the object.
Motion
If you want to have a clip move over the screen as part of your presentation, you need to use a Video Action.