How to add images at the end of an existing movie?
Use this command:

ffmpeg -i "video" -loop 1 -t 3 -i "image" -f lavfi -t 3 -i anullsrc -filter_complex "[0:v] [0:a] [1:v] [2:a] concat=n=2:v=1:a=1 [v] [a]" -c:v libx264 -c:a aac -strict -2 -map "[v]" -map "[a]" output.mp4


Replace the t value with the duration that you want the image to stay. I've supplied the common encoding parameters. Replace those as needed.

For an image sequence, use

ffmpeg -i "video" -f image2 -framerate n -i "image" -f lavfi -t 3 -i aevalsrc=0 -filter_complex "[0:v] [0:a] [1:v] [2:a] concat=n=2:v=1:a=1 [v] [a]" -c:v libx264 -c:a aac -strict -2 -map "[v]" -map "[a]" output.mp4


Replace the n in framerate with the rate that you want.