Because we need more sigs that involve balls... Signatures are images that appear under a user's post.

First, upload the image to something like photobucket. Copy the image's URL.

Then go to your profile/UserCP.

Go all the way to your signature input area.

Paste the image URL in a code like this.

[img]Paste%20URL%20HERE[/img]

Example:
[img]http://img87.imageshack.us/img87/2729/zacksigls6.jpg[/img]

Finally save your settings.

Alex hasn't invented any sig size rules. So just use a reasonable image size.