Well... Tigger and Grad are resident sigmeisters, they will knock up a good one for you.
If you already have one then it has to be hosted on a website. So that the message board can go grab it to display. If you have one online then you need to click the 'UserCP' and then 'Edit Profile'.
Then in the signature box you need enter:
{IMG}
http://mysite.com/myimage.jpg{/IMG}
(Change the { bracket for a [ bracket
)
The same applies for funny images you want to post. They must have a location on the web. You can get some free web space that you can use for this. The just click the IMG button while creating the new thread to insert the image.
To find some images go to:
http://www.google.com/imghp and use the keyword search
Hope some of this helps.
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Fug