Here's a more in depth example with BBCODE of what would happen if you don't replace the IMG tag.

1. Your current signature code looks like this:
   
   Code:

   Line 1
   [IMG]http://img208.imageshack.us/img208/1809/haruhirs5.png[/IMG]
   Line 2

2. It appears like this:
   
   Code:

   Line 1
   
   Line 2

3. After use of the IMG tag in signatures has been removed and replaced with SIGPIC; to have the same appearance as #2 upload the image:
   
4. Then use this code in your signature:
   
   Code:

   Line 1
   [SIGPIC][/SIGPIC]
   Line 2

5. If you don't do the above changes in #3 & #4 your signature will appear like this:
   
   Code:

   Line 1
   http://img208.imageshack.us/img208/1809/haruhirs5.png
   Line 2

If you compare #1 with #5 you can see what happened when the IMG tag was removed.

You can always upload another image for your signature, the SIGPIC tag will automatically use the new image.