Whatever text or image needs to be under the drawshade, you start with
open-bracket spoiler close-bracket, like this (spoiler) except using square brackets. Then finish by the same command but with a slash. You can see it by quote-replying any message that uses spoiler. Let's experiment to see if it's searchable.
Edit: whoops didn't work, lemme go find a quote
Edit again: There, no slash to start, slash to end
Edit again: and searching for poopyface works! We could also use the /hide command if we don't want guests to be able to show the content under cover. Harder to use, but mods could help edit posts to hide content, and we could all coach new posters.