Override theme image style to remove query string.

<?php

/**
* Override theme image style to remove query string.
*/
function THEMENAME_image_style($variables) {
// Determine the dimensions of the styled image.
$dimensions = array(
'width' => $variables['width'],
'height' => $variables['height'],
);

image_style_transform_dimensions($variables['style_name'], $dimensions);

$variables['width'] = $dimensions['width'];
$variables['height'] = $dimensions['height'];

// Determine the URL for the styled image.
$variables['path'] = image_style_url($variables['style_name'], $variables['path']);
// Remove query string for image.
$variables['path'] = preg_replace('/\?.*/', '', $variables['path']);

return theme('image', $variables);
}