Official ffmpeg documentation on this: Create a thumbnail image every X seconds of the video

Output one image every second:

ffmpeg -i input.mp4 -vf fps=1 out%d.png

Output one image every minute:

ffmpeg -i test.mp4 -vf fps=1/60 thumb%04d.png

Output one image every 10 minutes:

ffmpeg -i test.mp4 -vf fps=1/600 thumb%04d.png

The command you are using re-encode the video frames.
Add -c:v copy for avoid re-encoding.

Replace ffmpeg -f mjpeg -i input.yuvj422p output_%04d.jpg with:

ffmpeg -f mjpeg -i input.yuvj422p -c:v copy output_%04d.jpg

In context, "re-encoding" means that the each video frame is decoded into uncompressed video frame, then encoded (compressed) to JPEG image format.
The process of JPEG decoding and encoding (re-encoding) is "lossy" (quality is lost), and the output images are different from the input video frames.

FFmpeg default behavior applies re-encoding (even when the input and the output are both in JPEG format).
In order to avoid re-encoding we may add -c:v copy argument, which forces FFmpeg to pass the input stream/frames to the output as is (without decoding and re-encoding).

Testing:

Building synthetic MJPEG video (used as input for testing):

ffmpeg -y -f lavfi -i testsrc=size=192x108:rate=1:duration=10 -vcodec mjpeg -pix_fmt yuvj422p -f mjpeg input.yuvj422p

Size of input.yuvj422p is 51K.

Converting MJPEG to JPEG images without -c:v copy:

ffmpeg -f mjpeg -i input.yuvj422p output_%04d.jpg

Total size of the 10 JPEG images is 44K.

Converting MJPEG to JPEG images with -c:v copy:

ffmpeg -f mjpeg -i input.yuvj422p -c:v copy output_%04d.jpg

Total size of the 10 JPEG images is 51K (same size as input.yuvj422p).

Just mux the images

You can simply mux the JPG images to make a video:

ffmpeg -framerate 30 -i input%03d.jpg -codec copy output.mkv

Note that if you omit -framerate then a default of -framerate 25 will be applied to the input.

Lossless optimization

You can use jpegtran to perform lossless optimization on each frame which may provide significant file size savings:

mkdir outputdir
for f in *.jpg; do jpegtran -optimize -copy none -perfect -v "$f" > "outputdir/$f"; done

Now mux with ffmpeg as shown above.

Checking that it is actually lossless

The framehash muxer can be used to compare the unique hash of each frame to ensure that the result is truly lossless:

$ ffmpeg -i input%03d.jpg -f framehash -
stream_index, packet_dts, packet_pts, packet_duration, packet_size, hash
0,          0,          0,        1,   460800, 29bcc2db3726c7dfec1826c5740f603f
0,          1,          1,        1,   460800, b5fdc23d93cbd043dc2b9290dc8378f0
0,          2,          2,        1,   460800, ee0709942f24b458fd2380d134dcb59d
...

$ ffmpeg -i output.mkv -map 0:v -f framehash -
stream_index, packet_dts, packet_pts, packet_duration, packet_size, hash
0,          0,          0,        1,   460800, 29bcc2db3726c7dfec1826c5740f603f
0,          1,          1,        1,   460800, b5fdc23d93cbd043dc2b9290dc8378f0
0,          2,          2,        1,   460800, ee0709942f24b458fd2380d134dcb59d
...

In the above examples each associated frame for the input and output share the same hash ensuring that the frames are identical and that the output is lossless.

Also see

• FFmpeg image file demuxer documentation
• FFmpeg Wiki: How do I encode single pictures into movies?

Use -qscale:v to control quality

Use -qscale:v (or the alias -q:v) as an output option.

• Normal range for JPEG is 2-31 with 31 being the worst quality.
• The scale is linear with double the qscale being roughly half the bitrate.
• Recommend trying values of 2-5.
• You can use a value of 1 but you must add the -qmin 1 output option (because the default is -qmin 2).

To output a series of images:

ffmpeg -i input.mp4 -qscale:v 2 output_%03d.jpg

See the image muxer documentation for more options involving image outputs.

To output a single image at ~60 seconds duration:

ffmpeg -ss 60 -i input.mp4 -qscale:v 4 -frames:v 1 output.jpg

To continuously overwrite/update/save to a single image

Use -update 1 image muxer option. Example for once per second from a live streaming input:

ffmpeg -i rtmp://input.foo -q:v 4 -r 1 -update 1 output.jpg

Also see

• FFmpeg FAQ: How do I encode movie to single pictures?
• FFmpeg Wiki: Create a thumbnail image every X seconds of the video

The quality of that command's output is bad for a few reasons:

• It is encoding using the MPEG-1 codec, which is quite outdated.
• You are not setting the bitrate, so it is coming up with its own guess, which is probably inadequate.

Try something like:

ffmpeg -f image2 -i image%d.jpg -vcodec mpeg4 -b 800k video.avi

for mpeg 4 video or:

ffmpeg -f image2 -i image%d.jpg -vcodec libx264 -b 800k video.avi

for H.264 video (You will need to have libx264 installed for this to work). You can play around with the bitrate because it depends on the size of your frames what bitrate you will need. Also, running ffmpeg -formats will display all of the output formats and codecs if you want to experiment more.

See the ffmpeg documentation for even more options.