Why This is Needed

Or would at least be favoured

People who make spoileriffic images are still lashed with comments of "I read this and it ruined my movie/game/etc!" even when putting SPOILER SPOILER in their title

The fact they put SPOILER SPOILER in the title shows that they're trying to filter anyway, and a tool would serve this purpose better

Some use the mature tag, but now that mature deviations are actually HIDDEN FROM BROWSING unless allowed (I feature I personally don't like, there should be middle ground - maybe I don't mind mature gore but hate mature nudity for example and want to excercise my OWN common sense) this actually HIDES THEM from young/conservative users and AUTOMATICALLY DISPLAYS THEM to older/less bothered users.

Some people are using GIF files and making the preview a warning image, but
- - - - ...many categories don't allow GIF files due to past misuse
- - - - It limits the colours usable in the main deviations
- - - - This option isn't even open to literature (perhaps the fanfic/etc is set after events in the original story and references them)

Whether a filter is provided (like the mature tag) allowing to always show or hide would be up to dA, but I'd recommend it as always tagged to prevent people who don't mind spoilers for Dragon Age not getting upset when they end up being shown spoilers for Mass Effect 2 as well (for example). Display deviations on a case-by-case basis

Alternatively, spoiler images could have a user-made preview to give more indication on the nature or the spoiler, but I think this would be easier on staff and artists alike, as well as cleaner.

Alternatively

You can provide this as a generic "user has opted not to use a thumbnail for this image" to make it useful to all galleries (though if I'm not mistaken Fan Art is actually a reasonably large portion of dA's intake), but then of course you run into the old issues of why GIFs where disabled in almost all categories.