You can't actually embed pictures with posts. But, in the top right-hand corner of all posts is a "Forum Attachments" link. This link will bring your window down to the bottom where you can click another link to "add a file".
Once a file is added, right-click the file to copy the url address ("copy link" or "copy shortcut" in different browsers) and paste it in the following code:
The code I use in my forum signatures is the following:
[[image http://tibasicdev.wikidot.com/local--files/home/tibasicdevsig4.png alt="TI-Basic Developer" link="http://tibasicdev.wikidot.com"]]
This links to the site portal page when it's clicked, and displays "TI-Basic Developer" when you hover your mouse over it.