Can't figure out how to make zoom effect.

[Bushdoctor]

Good day,
I'm using X5.

I would like to create a certain zoom effect, but I can't get it done.
I'm working with graffiti photo's. I want to show a wall with several different pieces on it.
After a few seconds, a photo showing just one of those pieces appears in an overlay.
It is placed at the same spot where its background-twin is located and its resized to match its appearance.
That small overlay now has to grow in size so it eventually fits the entire screen.
It is however important that, during this 'zoom effect', the background photo remains visible.

I have tried to work with the Picture-in-Picture effect from NewBlue, which sounded like the solution.
I managed to get the entire thing done, except for making the background visible during the effect.
The resized overlay picture is surrounded by a black screen that covers the background photo and I can't figure out how to make that black surface transparent.
It ís possible to make the entire overlay transparent, but that also affects the black color in the photo, which obviously messes it up.
Anybody got any idea's how to achieve the effect I'm looking for?

Greets and thanks,
Bushdoctor.

[Bushdoctor]

Holy smokes.. I was almost there!
In the overlay transparency, all I had to do was select the Mask Frame instead of the Chroma thingy (Not sure what the English names are).
It allowed me to select a box around the picture while making the rest transparent.
Worked out great!

[lata]

This one

click image for larger view

[Bushdoctor]

Yessir, that be the one. Works like a charm.
Thanks.