Add blur effect to images in drupal 7?
Sure, you'll need to write a custom module and implement hook_image_effect_info() to register your effect with the system, e.g.:

function MYMODULE_image_effect_info() {
  return array(
    'mymodule_blur' =&gt; array(
      'label' =&gt; t('Blur'), 
      'help' =&gt; t('Blur an image.'), 
      'effect callback' =&gt; 'mymodule_blur_effect',
    )
  );
}


Then implement your callback function (mymodule_blur_effect()) to manipulate the image. Try following the function trail from image_resize() to get some inspiration on exactly how to access the image stream data.