ISF is a vector format. Math Input Panel API also require strict stroke order for better symbol recognition. PNG is a raster image format, that don't contain any stroke-data(no vectors, no stroke-order). It's very easy to convert ISF to PNG, but reverse conversion is hard, and there no built-ins to do this.
To convert PNG to ICF must somehow convert pixels to vectors(write own little vectorizer using center-line tracing algorithm for example), and correctly order strokes in vector result(from upper-left to upper-right used used in most languages, except RTL like arabic).