Very simple way without any code knowledge..
1- Go to “Pages” (in the left sidebar of your admin)
2-
a: Create a new page
b: remain the title field blank
c: click insert media and upload the image into page. You may enlarge the image as per the page width etc.
d: publish the page
3- Go to Settings > Reading (in the left sidebar of your admin)
4- In Front page displays Select A static page (select below) . From Front page: dropdown menu select the page (has no title) as your homepage.
Enjoy:)