for page in 0 1 2 3 4 5 6; do
  for num in 0 1 2 3; do
    convert -density 300 uebungsbeschreibung_sitzen.pdf[$page]\
      -alpha remove\
      -crop 2500x670+100+$((470+$num*689))\
      -quality 85\
      uebung-$page-$num.jpg;
  done;
done