When you find a picture you want to put into your signature, here is what you do:
First you need the URL address of the picture. To find the URL of an image, right-click on it. Then click on "properties". A box will pop up, and about halfway down it will show the URL address. You can just highlight it, right-click on it, and click "copy". Then go into your profile by clicking "my profile" at the top of this page. Scroll down to the signature box, and type in . Put the URL address of the picture you chose right smack in-between .
That should do it.
The little pictures that move are called gifs. There are tons of them available for free. Just search the web using words like "free gifs" and stuff like that. You should be able to find lots of sites that have them. Then, just follow the same directions I posted in the first paragraph. Hope all this helps!