A conversion from .mp4 multimedia file to a .png picture is usually used to a create a video like sequence of images frames from actual video file.
A lot of users had success with FFmpeg program and using the following command lime sequence:
ffmpeg -i input.mp4 -r 25 -f image2 imagefolder\images%05d.png
You can find more info about this on some of the FFmpeg tutorial websites.
There may be other programs capable of this conversion. However, most of them achieve this with poor results.