Compositing alpha overlay In the compositing layout the alpha over won't work. The background stays grey. How to make it transparent so that the background can be seen? 

I followed this tutorial here: https://www.youtube.com/watch?v=l26kp1ETRk8



I wouldn't mind rendering just the image, and I did, but the strip alpha overlay doesn't work either, because it's a grey colour.
Go to the render tab -> Shading and change the Alpha from Sky to Transparent.