Using the properely thumbnail in Facebook pages:
See the demo here -->
http://zocomac.com/lacie-2big-network-2tb-20110810296/ (use the "like" FB button and see the thumbnail on your wall)
You need to include something like this in your classipress/header.php
<?php if ( is_single() ) { ?>
<meta property="og:image" content="<?php rb_get_image_url_feat($post->ID, 'thumbnail', 'preview', 1); ?>"/>
<!-- you should find another function instead of "rb_get_image_url_feat" for this since this what I've already developed for myself -->
<?php } else { ?>
<meta property="og:image" content="http://your-image-url"/>
<!-- this is the image in case there is no image included in your ad -->
<?php } ?>
Hope you enjoy this!
Best regards,