I can not put large background image on my home page

Good morning, I write to you, because I’ve been trying to put a background image that occupies the entire screen of my web, when I enter it, and it’s not possible for me. The most I’ve managed to do is put it in the logo area.
But besides being extremely small, and that the photo is of quality, it is not where I want to put it.
I do not want to put it in only one part, but I do background background when entering the Home of my web.

On the other hand and this is another issue, I’m not working very well the Enfold theme, because every time I make a change, I have to wait several hours, or even days until I see it reflected, and as you will understand, that greatly impedes the Workflow and is very cumbersome for me.
I do not know what the problem is that happens, but I need a solution.
I already talked about it with one of your colleagues, and they told me that it will delete cookies and cache, but the same thing still happens to me. I beg you to answer these two questions, thank you in advance!