To embed, hit 'go advanced' and them click the little image icon (looks like a grey mountain on a yellowish background). Enter the url of your image there and it should embed.
I upload my images to my iDisk, I'm not sure what the url is if you upload it to mac-forums.
If you click the line that says 'Version: 10.4.7' you can toggle between serial nr., os x build nr, and os version.
BlooGrape: If someone has your serial number they can't really do all that much, but it could be abused on the phone or online with apple support or something.
E.g. If your iPod headphones are broken, you can enter your serial number and request a replacement.