Create a page via WP Admin > Pages > Add New. In the page put whatever image you want. Save the page (i.e. click Update). In the WP Admin go to Setting > Reading. By default "Front page displays" it's set to "Your latest posts." Change that to "A static page." Next to "Front page" there's a combobox. Click on it and select that page you just created. Then click on "Save changes" at the bottom.