How do I get a pic under my posts?

That is called a Signiture! A.K.A Sig, you get it by clicking the profile tab.

look above the profile panel for these
Information | Preferences | Signature | Avatar | Friends and foes | Notifications | Topic(s) being watched | Favourites

Click Signiture.

Now, Upload an image on to a image uploading site like photobucket, once uploaded copy the code that has [IMG] in it.

Paste that code where your Signiture is! this may not work for users under a week old, as we do not allow users under a week old to post outgoing links, which Photobucket or many other Image uploading sites have in there code.