Different people have different interpretations of what's a spoiler and what isn't, though.
I posted a screenshot of Egomassive's "A Knytt in Time" that I didn't think was spoilery, but he felt it was extremely spoilery and asked me to at least spoiler-tag it. Similarly, Razzorman put up a screenshot of yohji's "Moonlight" that yohji felt was too spoilery, though Razzorman presumably hadn't thought it was spoilery when he posted it.
It's likely that, even if the player thinks their screenshot isn't 'really' spoilery but puts it behind a spoiler tag just in case, some level designers are going to prefer that the particular screenshot the player chose not be posted even behind a tag.
I think a thread like this is a good idea -- ironically, I was only motivated to finally download & play "Moonlight" after seeing the aforementioned screenshot, since I'd been expecting it to visually resemble "Pestilence" which, while a brilliantly-conceived level, is kind of hard on the eyes (though that really is, I think, a necessary component of "Pestilence", so this isn't a criticism or complaint). The logistics of it seem potentially problematic, though.