- preg_replace: documentation ( source)
<?php
$_content = '<br><div><a href="http://activistpost.net/organicfood.html" rel="nofollow"><img src="http://lh5.ggpht.com/_qLAIskTQXUc/TMcjgprXEmI/AAAAAAAAEE0/0_pPXQypggE/468x60-set1.gif"></a></div>';
$_content = preg_replace('#<img ([^>]*?)4\d{2}x6\d{1}[^>]*>#i', '', $_content);
echo $_content;