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/yourpicsname">

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/yourpicsname">
</CENTER>