The HTML for putting a picture in your auctions is-

<IMG SRC="Your picture URL goes here">

For example-

<IMG SRC="http://www.yourimagehost.com/yourname/yourpicture.jpg">

There is no closing tag needed when posting an image.
If you'd like a more detailed explanation on how to post images Click here

To make your picture appear in the center of the page type-

<CENTER><IMG SRC="http://www.yourimagehost.com/yourname/yourpicture.jpg"></CENTER>

You will find a HTML Practice Board by clicking here