To be fair, if people didn't want to be spoiled, they wouldn't look at the image which is clearly tagged spoilers. Which being posted here is just as bad.
You still have to use the tags in spoilery comments, though. Someone might have spoilers blacklisted and thus wouldn't see the image, but if they were in the Comments page and the text wasn't blacked out, like so, then they'd see it and be spoiled anyway, regardless of how vigilant users were in adding the spoilers tag to the image. That's not fair, which is why marking spoilers in comments is mandatory.
