search in result_array()

hi...
im returning db query as result array:

$sql="select image_name,image_caption from gallery_image_captions where gallery_id=".$gallery_id." order by image_name";
$query = $this->db->query($sql);
return $query->result_array();

now i have
image_name1 "image caption 1"
image_name2 "image caption 2"
image_name3 "image caption 3"
image_name4 "image caption 4"

how can i found in the result array a value by some key? i mean the function will get key (image_name3) and value will be "image caption 3"

thanks