The "M" of XLSM stands for macro. Macros are programs embedded in Office documents used to automate different tasks. Staring from Excel 2007, XLSX spreadsheets that contain macros are saved with the XLSM extension.
PNG or Portable Network Graphic format is a graphic file format that uses lossless compression algorithm to store raster images. It uses 2 stage compression methods. It is frequently used as web site images rather than printing as it supports only the RGB color model. So CMYK color images cannot be saved as PNG image.