First get logged in to a GCE instance via SSH. From there you can access the Storage with the help of a Service Account:

Step 1:  Open Command line of google app engine and enter following command.

          $ sudo gcloud auth login

Step 2: Login to gmail by copying the url for authentication and after login with gmail you will get the code. Copy the code and paste it and hit enter

Step 3: Now Write the following command

       $sudo gsutil cp /var/www/sliderVideos/thumb/201504100904091029698703.png gs://amuk

Write the following code in php to test, whether you are able to copy the files to directory to "amuk" directory created in google storage (Google App Engine)

<?php

$output = exec ('sudo gsutil cp /var/www/sliderVideos/thumb/201504100904091029698703.png gs://amuk');
echo "<pre>$output</pre>";
?>

Now you can see that you are able to access the storage account. Please have a look on the screen shots attached below