I'll take a partial stab at this. But first, I believe this rocket is not on a GTO trajectory (judging by the (relatively) high latitude view of the CONUS + Mexico, indicating inclined orbit). This is probably GPS III-05 (17 June 2021 16:09 UTC launch, image was tweeted ~4 hours later), TLE history for reference:
Not knowing the specs of the camera used, I ...
I believe I got the answer.
Let's check this explanation:
it's unintentional and an artifact of how the sensor works. Basically it still works as a much lower performance image sensor after the shutter is closed but before the stored pixel data is scanned out. Essentially photoelectrons can still get into the storage node.
Since Urban (1987) is available, I could look for the Details of IDC and FAST implementation missing:
What spatial predictor was used?
Simple scanline order pixel differences, or in other words a LEFT predictor.
What does "adaptive split pixel compressor" mean?
"Adaptive" here means they spilt the image into blocks and use different ...
I tried to look for a publicly accessible link to the Urban (1987) paper that was referenced in @Organic Marble's answer. Fortunately, it is publicly accessible through the University of Arizona: Voyager Image Data Compression and Block Encoding
From pages 5 and 6, we see that:
the Image Data Compressor (IDC) is a universal noiseless coding compressor
It would be nice to look at Voyager image data compression and block encoding by Urban, but it's paywalled.
It's described in Space Data Compression Standards though, which says
Robert Rice of the
Jet Propulsion Laboratory devised an adaptive code for
the Voyager mission,6-10 which was extremely efficient in
terms of detecting changes in data entropy and ...
Are all those dots impact craters and if not what are they?
They are artifacts of some sort.
They could be salt and pepper noise, but I suspect not. If it was salt and pepper noise the image would also have lots of white dots, which it does not. These artifacts are instead missing data. The standard standard approach with regard to imagery errors traceable ...