An episode thumbnail shows a character who, as far as you knew, left the show two seasons ago. A film's artwork features a moment that only makes sense after the reveal. Neither is an accident in the sense of somebody being careless, and understanding why makes it clearer which complaints are worth acting on.
Why the artwork does this
Services choose artwork by testing, and Netflix has written about how it does this. Multiple images per title are shown to different groups, and the one with the highest engagement wins. Some services go further and vary the image by viewer, based on what that person has watched before, which Netflix describes as part of how its recommendations work.
A dramatic still outperforms a neutral one. A moment of confrontation gets more clicks than a portrait. The optimisation has no concept of what constitutes a reveal, so an image that spoils something is frequently the image that performs best, and the system selects it for that reason.
Episode thumbnails have the same problem in a sharper form, because they are often pulled automatically from a frame partway through the episode. Nothing checks whether that frame is a climax.
What you can actually change
Do not browse the episode grid. The single most effective habit. Start an episode and continue from the player rather than backing out to the list between episodes. Most spoiler exposure to episode stills happens while scrolling a season list, and simply not opening it removes the whole category.
Turn off preview autoplay. The video previews that start while you hover carry the same risk as the stills and are more likely to include dialogue. This is a per-profile setting on most services, and we list the paths in how to stop apps autoplaying at you.
Clear finished and abandoned titles out of the home screen. A show you are midway through keeps generating fresh artwork for the next episode on the home row. Removing titles you are not actively watching cuts down how much of that you see, and each service handles removal differently.
Use search rather than the home screen. Going straight to a title you have decided on avoids the promoted rows entirely, and on Netflix the genre pages do the same job for browsing.
What does not work
Worth stating, because these come up repeatedly and cost people time.
There is no setting on any major service to switch off dynamically selected artwork or to force a neutral poster. It is not exposed as an option.
Clearing your viewing history does not reliably produce different artwork. It may change which image you are shown, but not whether spoiler images are used.
Browser extensions that blank thumbnails work only in a browser, and most viewing happens on a television, where no equivalent exists.
Episode titles do it too
Less discussed than artwork and harder to avoid. Episode titles are set by the production rather than the service, and some of them name an event before it happens.
No service lets you hide episode titles, and because they are metadata rather than images no extension can blank them either. The same habit covers both: stay in the player and move to the next episode from the end of the current one, rather than returning to a list that shows every title and still at once.
The one that is genuinely unavoidable
Recommendation rows for a show you are watching sometimes surface later-season artwork. Nothing you can set will prevent it.
If you are deliberately catching up on something everyone else finished, the practical answer is to stay out of the browse screens for the duration and go directly to what you are watching. That is a real cost of the interface rather than a failure on your part, and it fits alongside the other habits in watching a show late without being spoiled.
How much this matters depends on what a service is measuring when it picks the artwork, and Netflix's own Top 10 ranks by views rather than hours watched, which is a useful reminder that the numbers a service shows you are the numbers it chose to show.
