they can be made with a paint programme IE photoshop paint.net etc. if you look on the siggy fourm there is a sticky on sig making, or there is a request thread where you can post pictures of your cats and put there names and someone will make one for you, and then post the sig in the completed sigs thread.
hope this helps, im not sure if you have to be a member for a few days or have so many posts before you can , i honestly can not remember. http://www.rosehawke.com/how_to/