ffmpeg can do what you want: https://www.ffmpeg.org/
If all the pictures in the video are shown for the same amount of time:
ffmpeg -i input -filter:v select='eq(n\,0)+gte(t-prev_selected_t\,5)',format=pix_fmts=rgb24 -vsync 0 output%03d.tif
This will grab the first frame plus one frame every 5...