Update Product Image

Product image do not get update when dropbox URL or server image URL is given.
“images” => array(
array(
“url” => “https://dropbox.com/s/fm6rgw4f9xtsg38/rivergrey-stead.jpg?raw=1
“is_default” => true
)
)
this does not work. If anything has changes w.r.t documentation, please let me know.
Thank you