You can always request a signature from one of the volunteering people in the
Graphics Showcase section.
Be sure to follow their template and whatnot.
After you've acquired your signature, copy the image URL (For example on Imgur).
-Go to "Edit My Profile", which can be found at the left (Quick Menu) under "My Profile".
-Go to "Profile Information" and scroll down to signature box.
Put the image URL into the following BBCode:
...And paste that in the signature box.
You can also use other BBCode such as center, colours etc.