The same way you post a pic on the forum.
In your sig options for your profile enter URL of the pic you wish to post. Make sure you do it with PIC tags not URL tags like your Victoria post.
Eg. PIC.............../PIC. Post the url where the dots are.Enclose the PIC like this [PIC] though.
If you have troubles I'm sure someone will fix it for you.
Feel free to PM me if you have trouble.
Its real simple, When posting an image use the 'IMG' Button, Like so then enter the URL of the source your wanting to add so it then looks like the following, [*img]www.thelinkhere.com[/*img] but remove the *'s and add the actual link you need.