Welcome to Pet Talk Laura! To learn how to post pictures, look in this thread:
Posting Pictures; class 101
It is very helpful.
To put a picture in your signature, you need to click on User CP.

^User Cp

Then you will see a screen like this:

You need to click on Edit Profile.

Then this will come up:

The arrow is pointing to where you need to type in what you want in your signature. If you want the picture to show up, as you can see you need to type, [7img] [7/img] (without the sevens) with the URL in the center.

I hope this helped!!