Forget masks. Keyframes in the Pan/Crop window are what you want to use.
In the screenshot example below, I created keyframes at the 3, 4, 7, 8, 11 and 12 second marks.
Start off at the beginning with your image at the desired person. Create a new keyframe at the 3 sec. mark. This will hold the shot for 3 sec.
Now go the 4 sec. mark and move things to the next person. A keyframe is automatically created for you. Create a new keyframe at the 7 sec. mark. Repeat this process as many times as necessary.
The timings I used here are for instruction purposes only and will vary depending on your needs.
Remember to drag the event out longer than you need to create the keyframes. It can be shortened when you’re all done.
Make sure your image resolution (in pixels) is high enough to allow you to zoom in to the point that you want.
What I mean is that if this is for a standard def project, zoom in the desired first spot and check the numbers in the pan/crop menu on the left side. If they’re much lower than 720 x 480, you need a higher resolution images as the quality is already suffering.
If these are images from a digital still camera, you’ll be fine as most of them are at least 2400 x 1600 pixels.
