Painting creator: resolution and transparancy

[Wouter36]

I tried to retouch some spots on the wall in my movie. I used painting creator and it looks fine in the PaintingCreator screen. But after adding the image to my movie, the brush has a white cloud around it.
I found out that it has to do with resolution. In Painting creator the resolution is 854x480, while the movie is 1280x720. So some pixels are interpolated, but not with transparancy, but in a grayscale. Does anyone have have solution? Now I have to open the image in Photoshop and repaint it into a 1280x720 image with transparancy.