As many people said, you shouldn't do this with a regexp. Most of the examples you've seen to replace the image tags are naive and would not work in every situation. The regular expression to take into account everything (assuming you have well-formed XHTML in the first place), would be very long, very complex and very hard to understand or edit later on. And even if you think that it works correctly then, the chances are it doesn't. You should really use a parser made specifically for parsing (X)HTML.
Here's how to do it properly without a regular expression using the DOM extension of PHP:
// add a root node to the XHTML and load it
$doc = new DOMDocument;
$doc->loadXML('<root>'.$str.'</root>');
// create a xpath query to find and delete all img elements
$xpath = new DOMXPath($doc);
foreach ($xpath->query('//img') as $node) {
$node->parentNode->removeChild($node);
}
// save the result
$str = $doc->saveXML($doc->documentElement);
// remove the root node
$str = substr($str, strlen('<root>'), -strlen('</root>'));