Go to your signature page
Signature
Familiarize yourself with the rules.
Quote:
Less than
500 pixels in size in
both width and height.
Total byte-size of less than
100,000 bytes.
Check your file size.
512 character limit for signature (including BBCode)
Keep in mind that this means the
total of all images in your signature.
You need to upload the image into a site like Photobucket, Imageshack, Tiny pic or other image hosting sites.
Do remember to set your Display to allow all images because some sites are not on Gaia's whitelist.
Get the image address or url then use this code.
[img]http://url of pic[/img]