Origin of brightness artifacts in intensity resolution compressed images
I've taken the original tiff and just set a low threshold for 1bit quantization (conversion to black and white):

so we can clearly make out that these artifacts look very circular, and placed in specific patterns around the image borders.
I think these are artifacts could stem from a resizing of the image with imperfect anti-aliasing, though I'm certainly confused by the shape and the lack of &quot;peaks&quot; along the x and y axes. That would actually suggest lens artifacts, as these tend to be circularly distributed. But I think we can rule this out, since the &quot;dot array&quot; at the edges of the picture clearly indicate this happened after cropping the picture digitally.
Another interesting find: quantizing at the other end yields this:

Looks like &quot;cathode ray burn in&quot; used to look – I would, however, not know how this could happen with a digital camera or a scanner. Maybe it's actually a feature of the face of that clock?