Produce 8x8 PNG tiles of all keyframes ('-skip_frame nokey') in a movie:

ffmpeg -skip_frame nokey -i file.avi -vf 'scale=128:72,tile=8x8' -an -vsync 0 keyframes%03d.png

The '-vsync 0' is necessary to prevent ffmpeg from duplicating each output frame to accommodate the originally detected frame rate.

Display 5 pictures in an area of 3x2 frames, with 7 pixels between them, and 2 pixels of initial margin, using mixed flat and named options:

tile=3x2:nb_frames=5:padding=7:margin=2