Adding buttons and test to an image

I want to add a call to action button and some text to the left-hand-side middle of the image you can see on my homepage http://www.assuredgoldinvestments.com. I would be much obliged if someone could give me some pointer. I already looked at the forum and found a related topic on ghost buttons and have added some CSS to my custom CSS, however the next line in that article simply says :

If we add that CSS to our site, we can now add a button with those colors like this:

Your button text

But I've no idea where to put that line. Also how do I overlay just some text on the image?