Heh, considering the fact that spoilers are left wide open now, there really can't be much of a "what if someone doesn't know it's a spoiler?" issue. After all, at least -someone- else is likely to be able to tag it as a spoiler, and it's always better late than never.
Some imageboards, shuushuu being my prime example, have this system in place, and it works alright. Having the real thumbnail blocked is annoying, I guess, but you would have a basic grasp on series that contain spoilers to know if you're interested in the image that way, you can see the tags by putting your mouse over the thumbnail. Having the "spoiler" image go away after a couple of seconds with the mouse might be somewhat annoying though; it slows you down. A feature could be implemented to have the "spoiler" image go away with a mouse click and let you see the thumbnail.
Of course, sometimes there's an issue saying, "is this a spoiler?" and "at what point does this stop being a spoiler?"
Some imageboards, shuushuu being my prime example, have this system in place, and it works alright. Having the real thumbnail blocked is annoying, I guess, but you would have a basic grasp on series that contain spoilers to know if you're interested in the image that way, you can see the tags by putting your mouse over the thumbnail. Having the "spoiler" image go away after a couple of seconds with the mouse might be somewhat annoying though; it slows you down. A feature could be implemented to have the "spoiler" image go away with a mouse click and let you see the thumbnail.
Of course, sometimes there's an issue saying, "is this a spoiler?" and "at what point does this stop being a spoiler?"
hum... The system you proposed could work well in the censorship like the blurring if we can tag it correctly. Now that's the touch part >.<
Radiosity
A new spoiler system
So, I was thinking about a spoiler system for uploads. It'd be kinda like the 'hold' tags in that it'd work on an image being uploaded (although it should also be possible to add this to existing images where necessary). Basically, you'd add 'spoiler' as a tag to your upload, and instead of thumbnailing the image as usual, the board would generate (or use an existing) spoiler image similar to the download image used for flash and stuff.
Couple of problems I can see so far:
- The thumbnail image won't be browsable in the usual way because it isn't actually a thumb of the image it represents.
First point can be semi-solved by people who DO know what something is tagging it with the spoiler tag, hopefully before any serious collatoral damage is done heh.Second point is a little trickier but I suppose could be solved by maybe generating two versions of a thumbnail, a blurry (or solid colour with 'spoiler' written on it) version to display normally, and the regular thumbnail which would display on mouseover (maybe with a second delay if possible to avoid accidental mouseovers as people move around the screen with the pointer).
How viable this is I don't really know (both coding and practicality) but I think it could be a useful feature, although admittedly moe is better about spoilers than other places like danbooru anyway. Still, it could serve as inspiration for other imageboards ;)
Also, Varion just came up with another interesting take on spoilers:
"if it was me i'd try and make the system allow you to mark tags as spoilers as well as images. so say someone uploaded an image of BIG SPOILER from symphonia (if you've played the game you'll probably have some idea of which spoiler this is referring to), if you hovered and got 'EVEN BIGGER SPOILER' that would be a spoiler, so tag that one, but people could then hover over, see 'tales_of_symphonia' and go oh, finished that, add it to their own personal whitelist or something"