To add background image for search area in homepage like on the demo page http://workscout.in/ you need to go to your Dashboard -> Appearance -> Customize -> Jobs Options -> Job Home Page Options:
You can upload/select image from media, for the best results, the images size should be 1920 × 720, after you select it, click “Choose file” then Save Changes. That’s all!
How to display the search box on my homepage?
Make sure your home page in Page Attributes as Page Template has set the Page with Jobs Search
You can do the same for Resume Search, but in Dashboard -> Appearance -> Customize -> Resumes Options -> Resume Home Page Options: