Adobe Premiere does green screening along with iMovie. You are taking chances with torrents, and they are illegal copies of the program. If you plan you buy in the future you can test drive it, you can get a 30 day trial from Adobe. If you plan to publish your material I would recommend buying the software, which unfortunately is expensive. I think Avid make something too. You might find it for free but there are some benefits to buying it. You will have a big learning curve, but it is fun.