iMovie 11: Cropping videos into different shapes

I really, really want to know how to crop a video clip into a circle or other shapes and overlay them onto the project. I know how to overlay clips but I can't find any way to crop the clip into different shapes. I researched for hours and I could not find an answer, I see people in YouTube that can edit and overlay multiple clips and add frames to them. I can imagine how to add multiple layers and there are answers on the internet. I am currently using iMovie '11.
Thanks in advance

You can only crop to the same geometrical dimensions...no circles, or other "shapes".....

codda said:

You can only crop to the same geometrical dimensions...no circles, or other "shapes".....

Click to expand...

Can I use other programs though?

Sure, if you want. Among other options, there is Adobe Premiere (or Premiere Elements), Final Cut X, and Avid.

Thank you so much

There's a lot of programs that you can use as an alternative on editing videos