First you have to have an image you like.
Make sure to fit the signature to the guideline size parameters.
Once you have the image on your computer and you know where it is on your computer (say in your my documents and my pictures folder) goto http://www.imageshack.us/
Click on the browse button on that site.
This will let you find where you have your picture on your computer. Click on the Open button.
Once you have found the picture and clicked on it click on the "Host It" button on the site.
This may take a few seconds to load the picture on their site.
Once the new page loads look for the section that says "Direct link to image"
Highlight and copy that whole address in the "Direct link to image" section.
Now come to this site, click on "User CP" link towards the top left side of the screen.
This will open another page and you will see a menu on the left side of the screen.
Click on "edit signature"
This will load a new page.
In the edit signature section type in...
Now put your cursor between the img tags and paste your address from the image that you copied.
so it will look something like
Once you do that, click on the "Save Signature" button on the bottom.
You should start seeing your signature image on old posts or the next time you post.