It's easy as two steps:

1) Paste this code into your shop<table border="0" cellpadding="0" cellspacing="0" width="400" height="400" background=www.sitepictureison.com/picturelink.gif><tr><td width="300" height="159" colspan="3"> </td></tr><tr><td width="1" height="128"></td><td width="298" height="128"><div style="width: 298; height: 128; overflow: auto; font-family: Verdana, Tahoma, Helvetica, sans-serif; font-size: 11px; color: #000000;"><br><div style="padding: 5px;">
Text goes here!
<br></div></div></td><td width="1" height="128"></td></tr><tr><td width="300" height="13" colspan="3"></td></tr></table></div>

2) Replace www.sitepictureison.com/picturelink.gif <--not an actual site, don't click--> with the actual link of the picture, like if you want just a plain shoyru picture on Neopets called shoyru, then you'd put a link to images.neopets.com/shoyru.gif. Replace text with your text...
and there you go!