It is highly unlikely you can directly convert png to stl, i.e. just take any random PNG pictures and convert them to STL format for 3D printing.
Nevertheless, some programs may offer tools to process "png slices" and recreate a 3D model from it, which can then be used for the purposes of 3D printing. Sometimes shades of gray are used to define the "plasticity" of the 3D model, but that works only for a very specific kind of .png images, but this is actually the closest thing to a png to stl conversion.