How to make a transparent form(or picturebox, panel) with C#

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Hello guys :)

I'm making a shooting flying game with C# nowadays.
We set Google earth as a background of the game by using form.
The form is called and Google earth is executed in this form.

but we have a problem in this part.
I wanna cover some image on Google earth, but it isn't allowed directly to that.
as we have surfed web, it is possible to make another form on a first form and let it make transparent. I thought we might attach some image on this transparent form, so we tried it but it isn't executed...

So if you have any example code about this or any experience that was succeeded before, please give me some explanation...