Welcome!
Here's an excerpt from the FAQ link:
In order to add an image to your message or your signature, you must have the image already available on a web server and reachable by a URL. This can be an image on your own personal web page, for example.
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You can do the same for your signature. Click the My Home option found on any page. Then, under the Main Configuration heading, click on "Personal Information, email, password...". Look for the Signature box, and enter your desired information, including any images/markup as above.
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Note: To keep the forums loading quickly for everyone, it is recommended that you do not exceed 35k for your image size.
Additional info. not from the FAQ:
You can create and use a photobucket.com account to upload the picture(s). From photobucket, after you've uploaded, you would just right-click on the picture and choose 'properties' to determine what the URL (web address) is for the picture. You can use the third tool-bar button while posting, to insert the image, putting that URL between the image tags. Use the advanced posting screen to get the toolbar.
Like this, except use 'image' or 'img' instead of 'tmp':
[tmp]http://www.whatever.com/some_picture.jpg[/tmp]
(if I had used 'image' or 'img', it would have tried to shown the pic)
I hope you get it to work... I always enjoy seeing pics.
Oh yea, use the "Preview Post" option to make sure it's working before you submit (I guess you were doing that already).
Hope this helps.
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Don't tighten it while you're fightin' it!