I like to use this way, so that whenever original image is not available you can load your default image that may be your favorite smiley or image saying Sorry ! Not Available, But in case if both the images are missing you can use text to display. where you can also you smiley then. have a look almost covers every case.
<img src="path_to_original_img/img.png" alt="Sorry! Image not available at this time"
onError="this.onerror=null;this.src='<?=base_url()?>assets1/img/default.jpg';">