This mod adds a new bbcode tag that'll hide what you wrap with it. It works with text and images. Clicking on the spoiler link, reveals the block. You can optionally add a text to the link too, ie: [spoiler=My Title]Spoiler goes here[/spoiler].
This tag can also be used for content that not everyone may want to see, like a nasty picture, for example. I thought of naming it [hide], but since many already have a spoiler tag, I decided I'd leave it at that for backwards compatibility.