How to extract a video frame using NVIDIA card
Since the video bitstream is being decoded on the GPU, CUDA upload isn't required.

Command with format conversion is

ffmpeg -hwaccel cuvid -c:v h264_cuvid -resize 1024x70 -i video_source \
       -vf "scale_npp=format=yuv420p,hwdownload,format=yuv420p" \
       -pix_fmt yuvj420p -color_range 2 -vframes 1 -y /tmp/frame.jpg