This code seems to work:
    PNGReadWriter
        putForm: ((TextStyle defaultFont characterFormAt: $T) magnifyBy: 16)
        onFileNamed: 'T1.png'.

This one produces a fully transparent image (at least in mac finder
and firefox renderers):
    PNGReadWriter
        putForm: ((TextStyle defaultFont characterFormAt: $T)
magnifyBy: 16) asMorph imageForm
        onFileNamed: 'T2.png'.

Though a FileList is able to display a preview of the image for both files,
importing with:
    Form openImageInWindow: (FileDirectory default / 'T2.png') fullName.
just leave a transparent image in upper corner...

While
    Form openImageInWindow: (FileDirectory default / 'T1.png') fullName.
works as expected...

Any clue ?

Nicolas