How do I convert .png file to .iso file?

You cannot and such a conversion would make no sense.
  • a .png is a type of graphics file, it contains one "drawing"
  • an .iso is a file containing a copy of a complete CD or DVD, including the directory, track structure, files, etc.
There is not enough information in a .png file to be able to convert it to a .iso file.
However if you just want to create an .iso file containing one .png file, there is probably CD or DVD burning software that will allow you to create an .iso file of the contents you want to "burn" to the CD or DVD and just not perform the physical "burn" but keep the .iso file. But it seems like a big waste of time and space as the .iso file will be many times larger than the original single .png file.