Basically, just post silly/goofy/random/etc-looking images, and then other people make up captions for it. And if necessary- If you get an image poster to laugh at a caption someone made for one of their images, they owe you a Rep.
So feel free to post an image whenever, and see if someone wants to caption it.
And for starters, the image from my current signature:
So feel free to post an image whenever, and see if someone wants to caption it.
And for starters, the image from my current signature: