Notice that in the line of text enhancement options that are listed above the message composing window, when you are writing a post, a new button has been added at the right end. It says "Spoiler"
If you click on that button, and then enter some text, and then click on the "CLose Tags" link, (just below the text enhancement buttons), the text will be hidden behind a message, after your post is submitted to the board. When viewing a post with hidden text, just click on the message that is displayed, in order to reveal the hidden text. If you click on the message a second time, the text will be re-hidden. You can also hide text when you are writing a post, simply by highlighting the text that you wish to hide, and then clicking on the "Spoiler" button. The other BBCode enhancements also work with the Spoiler code. Here's an example:
[spoiler]This Text Is Hidden![/spoiler]
Here's an example of an image that has been hidden by using the Spoiler tag:
[spoiler]

[/spoiler]
If you have any questions, please don't hesitate to ask.
Tex