Extracting good stills from video

I need to extract stillframes from video automatically when I compress and autopublish some .flv and .mp4 videos. It's pretty easy to configure FFMPEG to do this at a pre decided frame but I need to be shure that I don't get a blurred out image!

I think the best way would be to define an area lets say 1/3 into the clip an scan it for 50 frames or so to decide wich frame has the least movement. Does anyone know of an easy way to do this?