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?

403 error generally means there's something wrong with the management certificate you use for authenticating your service management API requests. I would recommend looking into that one first. Also if you could share your code, that would give us some idea about what could be possibly wrong.