ffmpeg formatting/image overlay

- source is 720x576 16:9 (2.39:1 visible)
- source gets formatted to remove letterbox bars and rescaled to 854x356 (square pixels):
ffmpeg -i input.mpg -vf crop=720:432:0:70,scale=854:356,setsar=1:1 -c:v libx264 -crf 16 -c:a libmp3lame -b:a 128k output.mp4

- I want to overlay an image (copyright info, an 854x356 png with alpha) for the entire duration of the source video

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Tried this without success:
ffmpeg -i input.mpg -i copyright.png -vf crop=720:432:0:70,scale=854:356,setsar=1:1 -filter_complex 'overlay=0:0' -vcodec libx264 -preset slow -crf 16 -acodec libmp3lame -b:a 128k test.mp4