After many tests, I found the solution. The format to specify is now image2 for png.
This works
def transform(:thumb, _) do
{:ffmpeg, fn(input, output) -> ["-i", input, "-f", "image2", "-vframes", "1", output] end, :png}
end
# or this too
def transform(:thumb, _) do
{:ffmpeg, fn(input, output) -> "-i #{input} -f image2 -vframes 1 #{output}" end, :png}
end
It’s possible to pass a list or a string, but -f image2 is the key.
For reference, this is where I found the way to correctly transform a video to a png thumbnail.
Thank You all for the helps 