Some PNGs render black in Safari

Has anyone seen a problem in which PNG images render totally black in Safari?

For me this happens on only one of my three Macs, only in Safari, and then only with some PNGs. So it's not related solely to my setup, nor solely to the PNGs I'm viewing. It must be some combination of the two.

I don't think this is the same as the PDF black screen problem. I've been there, and fixed it by calibrating my monitor. But that hasn't cured the PNG problem, which I believe is relatively new (within the last two or three months).

My three Macs are a 2009 Mac mini, a 2011 Mac mini and a 2015 MBP. All are running macOS 10.13.3 (yes, even the 2009 Mac mini). The 2011 Mac mini is the only one on which I've been able to reproduce the problem.

My test images are at https://stanhoe.org/history/maps/1905-map and the larger lightbox version that displays when you click on the map. On the Mac mini 2011 in Safari these consistently show as black, whether I view them on the website or download them first. Occasionally I see the correct graphic for perhaps a tenth of a second, and then it goes black (the PDF bug gave the same behaviour, almost as if the image is there but is overlaid by a black mask).

On the same machine those same images display fine in Preview, Chrome and Firefox. They are also fine in Safari on the other two Macs (and my iPhone).

I have 6 Safari extensions (1Password, Adblock Plus, DEVONthink, Shortly, Pocket, Translate). Disabling them makes no difference, and besides I run the same combination on the MBP with no problems.

On the same website (mine) is another pair of PNGs at https://stanhoe.org/history/maps/1639-map. They give the same results. The other maps in that section are JPGs, so they work OK. I probably prepared the PNGs with ImageAlpha, so it may be that this app created PNGs that are unusual in some way. But that doesn't really explain why they display OK in other apps and in Safari on other Macs.

Any ideas on how to troubleshoot further will be very welcome.

Charles