We have to be able to show images from all across the series.
If a different scantlator took over, their quality has to be shown as well. Furthermore, if there is an improvement or drop in quality in a group's releases, it will be useful to show this in the screenshots.
It's not that rampant of a problem. Even in your so-called examples, those images don't really spoil much of anything. They don't ruin any significant plot twist, none of the plot is spoiled.
Honestly, I don't even remember the sample screenshots after I start reading a series.