is there a way to assign the PNG file?
According to Dynamic Icons help, the only direct method is interactive (good ol' drag'n'drop, with the Icon type radio button set to "PNG"), not programmatic (e.g. via setup strings). Time for an RFE? ; )
Indirectly, you need to add your icon file to %BIGICONSPATH%, find "bigicons.txt" there and add a line like
"<MyObectID>" "MyBigIcon.png" to it under the [OBJECTID] section.