Drupal Image API - Skip GIFs

template.php

function THEME_image_style($variables) {

  // Skip GIFs.
  $ext = pathinfo($variables['path'], PATHINFO_EXTENSION);
  if ($ext == 'gif') {
    return theme('image', $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']);
  return theme('image', $variables);
}