Simple one, overlay an image at a particular time.
First I figured out the resolution of base.mp4 with ffprobe – there’s
probably a way to get only the res but I just dug through the output to find it
ffprobe base.mp4
The resize the image to fit the video. I didn’t overly care if part of it was cropped so I just resized it to the largest dimension of the video
convert cat.jpg -resize x480 cat_resized.jpg
Use the overlay filter, at position 0,0 when t is greater than or equal 4s
ffmpeg -i base.mp4 -i cat_resized.jpg \
-filter_complex "overlay=0:0:enabled='gte(t, 4)'" \
out.mp4