Origin of brightness artifacts in intensity resolution compressed images when compressing this image



(It corresponds to Fig2.20 from the book Digital Image Processing by Gonzalez and Woods and the original .tif document can be downloaded through this 
link

By reducing the intensity resolution with Matlab I obtain a similar image that exaggerates the following fluctuations brightness fluctuations in the background.


Just out of curiosity, does anybody know what the origin for those is? I am puzzled as to whether they originate 

at a physical level, like small stains on the lens that took the photograph, (although they suspiciously stop near the watch)

at the transition of the signal into digital form when the image is captured by the pixels of the camera for example

at some step of compressing/ converting the image from one format to the other to obtain the .tif image. Thanks for your help!
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?