Add login button to site header

Hi,

I need to add a login button to the top right hand corner of my site’s header. See http://www.genovo.co.uk.

Could you explain how I can best achieve this. I was thinking of uploading a button (image) to the media library and then adding this to the header and setting the link URL to the login page of my app, but I can’t see how I can achieve this.

Many thanks in advance.

Ed