Here's a thought, with the caveat that I DON'T KNOW IF WE CAN MAKE THIS WORK.
I found another site out there in the world
that has a NSFW tag. According to the description, it doesn't load the image until after the person clicks the link. This would also make sure that it's clear that this isn't a spoiler and is truly a way of masking the image.
Is that something that those against the spoiler route would be OK with?
(Other mods feel free to chime in on this as well, either publicly or privately.)