SOLVED

Thank you for your reply.

The thing is, it doesn't, work in my case, since I have exported 250 slides as PNG images. It is animation and not a still rendered image. It has no sense to manually fix background of all 250 slides. I have stayed up all night trying to resolve this, and I have found out that resolution must be higher. Much higher. I also tried exporting animation with green background and then I was using AfterFX to remove background. The difference is minor. The solution is to at least double the resolution size and so the rendering time also... :(