Seen this on alot of other forums and seems an awesome idea.
Basicly when someone wants to post a large picture in a signature or in a thread they can put it inside a spoiler box to minumise the space it takes up, then if someone wishes to view the imagine they click the "spoiler" button and then image becomes visable underneath. This also can be done to add extra information to a post that isnt relevant to the original topic.
Sorry if i explained this horribly, if you have no clue what im talking about ill take a few screen shots from other forums that use them and post.
Just fed up of scrolling through a 5 inch signature when ive seen it 4 times in the last minute.