Hi there. You can edit them by going to the top of the site where it says User CP and click edit signature or edit avatar. I think you have to have a certain amount of posts before you can have an avatar. You have to have so many posts to be able to post your own too but you can choose from the forums until then.
and hope you've already visited the Sticky's at the top of the page of posts, which have answers to your questions. If you need additional help, you could try clicking on any Moderator's name and sending an SOS. They are wonderful and more than willing to help. Hope you enjoy it here and we look forward to seeing and hearing all about your cat family!
You can ask our talented siggy makers to make you a sig. Just post in this thread with your request, along with links to large, high quality pictures of your kitties.