function img_size($img,$alt) {
 
  list($width,$height) = getimagesize($img);
 
  $maxwidth = 140; //set max width here
 
  if ($width > $maxwidth) {
	$i = $width - $maxwidth;
	$x = $width - $i;
	$percent = $maxwidth/$width;
  }
  else
  {
    $x = $width;
    $percent = 1;
  }
 
  $y = $percent * $height;
 
  $img_src = '<img src="'.$img.'" width="'.$x.'" height="'.$y.'" alt="'.$alt.'" border="0" />';
 
  return $img_src;
}
 
echo img_size('http://somesite.com/image.jpg', 'some alt text'); 
//result <img src="http://somesite.com/image.jpg" width="140" height="(proportionate to 140)" alt="(alt)" border="0">