Hello Carlos!
Here's how you get your bubble.
1) Insert your still or video clip on a PiP track(work out at which point you want it to appear, how many seconds or minutes into your video).
2) Click on "Modify" and select "Mask", you'll find a considerable variety of mask shapes, choose which one you want to use and it will be applied to your PiP image.
3) Now you need only decide on how the image will appear, be it fade in/out or slide in/out.
That advice is for another time if you need help there. But for the moment,Steps 1 and 2 will give you what you want.
Cheers!
Neil.