From Webkinz Insider Wiki
Spoilers are commonly used in many online forums including WI. They allow information to be hidden until the user wants to see the information. Common uses for spoilers are outlined in the Frequently Asked Questions (FAQs) below.
Below are some frequently asked questions about spoilers.
Q: What is a spoiler?
A: A spoiler is a text "hider". It will hide your text, when you click "show" you will be able to see the text that is in the box.
Q: When I open the spoiler, I see another spoiler! Help?
A: That is OK. You can put spoilers INSIDE each other. This is called nesting spoilers. Nesting spoilers are not encouraged on WI. Nested spoilers can be difficult for the staff to moderate. It is highly encouraged not to nest your spoilers.
Q: What are spoilers used for?
A: Spoilers have multiple uses. You can use them to hold links, keep old updates on your thread, or organize your GE!
Q: How can i make spoilers inside each other?
A: You shouldn't place spoilers inside spoilers. So because of this reason we will not be covering how to do it in this guide.
How to add images inside spoilers
Lets say you want to put a picture of the new recipe you solved in a spoiler. [spoiler] (Picture goes here) [/spoiler]
Copy and paste the picture after the first brackets.
If you do not get how to do that, and want spoilers, have no fear!
There is another way. When you go to make your post, or click "Go Advanced", there is a little black box that says spoiler in it.
When you have what you want in a spoiler, highlight it, then click the Spoiler box. It will wrap the text in a spoiler. 
(Last Edit Nov. 24th, 2009, 4PMKT.)