First you have to save the picture on a photosharing website. Many people here use photobucket.com.
After you have uploaded and saved a picture on photobucket:
Go to Photobucket
Find the photo you want to insert
Hover your mouse on the photo you want to use.
Some information will appear under the photo.
Under the photo, click in the IMG CODE box
If you have a Mac, you will then need to right click in the box, and click on COPY
Log onto the disboards
Click on CP to get to the Control Panel
Click on EDIT SIGNATURE
Click on the place where you want to insert the photo
EDIT
PASTE
Click on PREVIEW SIGNATURE to see if you like it
If you like it, click on SAVE CHANGES.