Do you mean pictures that follow all of your posts? That would be a signature, in which case you somehow managed to stumble upon the right thread. You can add a signature by going to your profile page, clicking on "Edit my Profile", going to the "Signature" tab, and uploading an image or even text in the given space.
If you're just looking to add a picture to a post, click on the little square with what appears to a tree in it. If you mouse over it, it should say "image". It's right between the grayed out "unlink" button and the two blue arrows "code" button. You can add whatever picture you want, given it's not ridiculously large and that it's been uploaded online and has a url.