You need a photobucket account or similar. Upload your pictures and it will give you the link to use in various formats. Although I click on the picture to open it in a larger view, then right click, select properties and copy the long file name.
Then back to the forum:
In the "switch to full reply" box click on the 4th icon from the left, above the text bow, next to the envelope.
I think there is more straight forward answer in the FAQs though.
ETA from the FAQs:
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. To place an image within a message, simply use the following Markup Tag:
[image]
http://www.url_to_image.com/image_name.gif[/image]
For example, if you have an image called cateye.gif and its available from your own website at
http://www.mywebsite.com/pics, then you would use the following image markup:
[image]
http://www.mywebsite.com/pics/cateye.gif[/image]
You can do the same for your signature. Click the 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.
Note: To keep the forums loading quickly for everyone, it is recommended that you do not exceed 35k for your image size.