msk is the native vfp mask method to make a bmp picture transparent.
it consists in painting in black all areas of the picture that we want to be displayed exactly as they are in the bmp file and leaving the areas to make transparent as white.this uses mspaint for ex as images editor

for PNG the transparency is already built in.putting them on a form : they are transparent(not all) but you can choose.
if you need modern transparent pngs go to:
https://www.iconfinder.com/free_icons

for transparent BMPs as the code pointed makes,the transparency is built in, no need to msk masks.
try the code (need gdiplusX library to download at vfp codeplex) , build some bmps and upload on a form
an see if its transparent.
the doubt is always cleared by testings.