Although most movie poster images are of a gratifyingly high quality, a number of them seem to have an image of (arguably) lesser quality locked in first place. This may be because they were the best available at one time, and there is of course a strong element of personal preference. How is this selection made? It seems to be more than just community likes/dislikes. Is it partly a matter of which images were uploaded first? What is the difference between the "primary" flag, and when an image is "locked"? Are locked or primary images still affected by community likes/dislikes?

I have read the Contributions Bible, and can find no explanation there.

Also, is it fair to say that the posters that were used to promote movies when they were originally released are preferred to the more recent covers used for their releases on optical media (or streaming). It is possible I notice this more because I have many older and classic movies in my collection.

If it is helpful, I can provide a list (with links) to those movies that may have such anomalies in their precedence of poster images.