Your Signature (sig), is the text/image, which appears at the bottom of every post you make. Here you can add a favorite quote, an image, or as Rimmer suggests, useful information.

To add text:

At the top of any page, click My Controls.
On the left, under Personal Profile, click Edit Signature.
In the open text box, type in the information you want to be displayed in your Sig.
Click the Update my Signature button.

Note:
Signatures are limited to 5 lines, or 2000 characters, whichever comes first.
If your signature is larger than the allotted size given, or deemed unacceptable, you will be requested to adjust your signature.
Failure to comply, will result in the removal of your signature.


To add an image:
The image you use for your sig, has to be hosted off-site (image hosting site, personal web site, etc.)
A couple of FREE image hosting sites;
photobucket
imageshack
freeimagehosting
A Sig image can either be static, or animated..

At the top of any page, click My Controls.
On the left, under Personal Profile, click Edit Signature.
In the open text box, above, or below the text, if present, insert your hosted image, using Image tags.
Click the Update my Signature button.

Note:
You are allowed only one image, in your signature.
Images can be no larger than, 500 pixels wide X 90 pixels high.
If you have more than one image, you will be requested to remove one.
If this is not done in a timely manner, the staff has the right to modify your signature, to abide by these rules.


Before doing any of this, it's a good idea to clear your browser's cache (Firefox), or your Temporary Internet Files (IE).
Failing to do so, sometimes causes you to not be able to see the changes, right away.