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>> I thought that something like this may work:
>> ffmpeg -r 1/5 -i y-01.png -r 1 -i y-02.png -c:v mpeg4 -vf "fps=2,format=yuv420p" out.mp4
>> It does not. The first image is displayed, only.
>> Kind regards from Poland,
>> zb.

zb,

You could create a short, temporary intermediate video for each frame:

ffmpeg -r 1/5 -i y-01.png -c:v mpeg4 -vf "fps=2,format=yuv420p" out.mp4


ffmpeg -r 1/10-i y-02.png -c:

v mpeg4 -vf "fps=2,format=yuv420p" out.mp4

5seconds for the first and 10seconds for the second ( and so on and so forth... )

Then concatenate each video:
FFmpeg FAQ: How can I join video files?
FFmpeg Wiki: How to concatenate (join, merge) media files

This will allow you to do what you need to from the command line but this is a very awkward and cumbersome way. Probably be much easier to use a video editor.