No hace falta usar regexp:
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title></title>
</head>
<body>
<div id="pp">contenedor de la respuesta</div>
<script>
response='ssssssss<img alt="" src="images/px" style="width: 1px; height: 1px;"/>fffff<img alt="" src="images/px" border="0"/>sssssss';
temp=document.createElement('div');
temp.innerHTML=response;
vec=temp.getElementsByTagName('img');
for(i=0;i<vec.length;i++)
if(vec[i].src.split('/').pop()=='px')
vec[i].parentNode.removeChild(vec[i]);
document.getElementById('pp').innerHTML=temp.innerHTML;
delete temp;
</script>
</body>
</html>