Assuming you have the picture on a web site used to store the picture, you place it in your signature the same way you would place it in a post.
- Click on "User CP"
- Click on edit signature (or something like that).
- A box will come up that looks very similar to a posting box.
- Place all the text in your signature and then click the place you want the picture amongst that text.
- Click the yellow "Insert Image" icon.
- In the dialog box that appears insert the url of your picture.
- Preview the signature to make sure you got what you wanted.
- Submit the signature.