Putting your renders in spoilers like this, leads many users who are browsing through threads rapidly to skip your art, friends...fortunately (for me!) this really cool one one was spotted and commented on right above, otherwise I would have missed it...Oh well, not complaining of course, but I don't understand why artists sometimes put their work in spoilers, other than for the surprise effect
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I used to use the spoiler tag a lot, but since most other artists around here haven't been, I essentially gave up. For a while I'd put the NSFW stuff under a spoiler tag, so that people browsing in a 'public' environment could keep the naughty bits hidden until it was safe to view them, but since other artists haven't bothered to do this, I gave up. Note that someone recently complained about one of my renders showing off bestiality and wanted it behind a spoiler tag, after I'd given up on spoiler tags, but that person hasn't posted a lot lately...
Also, the spoiler tag helps with loading times, so that you won't be waiting for all of those images to load if there are a lot of images on the page, allowing the user to scan the page before opening up the spoiler tags, but again if most people aren't using them, kind of defeats the purpose. Thumbnails can help with that as well, but then people might not bother to look at all of the images in the thumbnails, which can be good or bad.
I usually only post one image at a time since I'm slow at making renders, so I don't usually bother with thumbnails, but then I almost always post 1080p or smaller images. 4K images take a lot longer to load, which might be a good time to do the thumbnail thing.
So yeah, the third option you mentioned, the surprise effect, that's also a valid use, but as you pointed out, then you run the risk of people not seeing your art, and since this is an art thread...