Heikki, here are 3 examples (using the same image each time) of what I do in this situation.
#1. Add the same image on a lower track.
#2. Add the medium (or whatever strength you want to use) Gaussian blur FX to the lower track.
#3. Open Pan/Crop on the lower track, right click and choose Match Output Aspect. This will make the blurred image fill the frame.
#4. Using Pan/Crop, adjust the position of the blurred image to your satisfaction.
#5. This is optional but I often add a Border FX (I made it smaller in these examples) to the top track to make the image stand out from the lower track.


