How can i crop a frame sequence with ffmpeg?
FFmpeg's image sequence demuxer has a start_number option, so if you want to skip img_0749.jpg and start from img_0750.jpg, you would start your command as follows,

ffmpeg -start_number 750 -i img_%04d.jpg ...


See the docs for the image2 demuxer at http://ffmpeg.org/ffmpeg-formats.html#image2-1