I realised it must be white on transparent (not on white) after I posted that last comment.
Interestingly, if I highlight the blank images from earlier, then drag them onto a darker background, I see concentric rhombic patterns. Do you see them? I'm not sure if it's just a strange facet of the way my laptop screen works.
I've also experimented here with posting pictures converted into ascii characters (displaying them in white again). But the forum seems to skew the image when displaying them, so I've given up bothering with that idea!