Hi Grace,
Just a tip, you should not put too much focus on the background-image because it is only for cosmetics. The user should have the focus on your content (form), not on the background-image. Another thing, no matter what size of your background-image it will not fit to everything, as you realize your page will be viewed on different device/screen sizes, a vertical/portrait oriented image will fit nicely on a portrait oriented device, but its not going to fit nicely on a landscape oriented screen.
I understand you want your contact page to look nice. But take a look on this example page (https://eagleheartdynamic.com/test-contact/) this is what you want to achieve, right? But it did not do justice to the beautiful Eagle image. If you want that Eagle image to put on display/focus you should redesign your layout, maybe do a 2 columns Eagle Image on the right, and form on the left. Hope this make sense.
Because it is easier to do it in Cornerstone. You can do that same Layout in your current contact page if you dont want to use the Cornerstone, but you need to learn how to write custom CSS.
To learn more custom CSS (when your schedule allows), you can inspect your site using Chrome’s developer tools to see the styling that is applied to an element and then using custom CSS to override it. You can find more info on how to check for CSS selectors here. Then information about writing your custom CSS here.
Hope this helps,
Cheers!