Getting 403 Forbidden on REST API request

I'm new (Trial) user and just testing Azure how it handles REST API services. As basic example I want to list OS images. So I'm using GET with my Subscription ID and x-ms-version:2013-03-01 in Header, just like explained here:

http://msdn.microsoft.com/en-us/library/windowsazure/jj157191.aspx

I'm always getting Error 403. Forbidden access.

The Subscription ID is from Settings panel in Azure portal.

Could it be something obvius I'm doing wrong here?