You can add a picture or quotes by going to your User CP and then down on the list go to Edit Signature.

When you click Edit Signature it will take you to a page where you can add
things.
For adding a picture you can scroll down on that page and use the browse button to find pictures on your computer or add a web address for an online image.

main thing is to stay within the size limits -
[Note: The maximum size of your custom image is 500 by 200 pixels or 24.4 KB (whichever is smaller). ]

If you need more detailed help after trying a few times let us know.