When you connect to your site with your FTP program, go into wp-content > themes > Tiga theme, or whatever the name is of the theme you're altering, and upload your header.php page (assuming you've added the code you need, of course) to overwrite the current page of the theme.
Still inside the same folder -
Next - Depending on what FTP program you're using, you may have the option to "make new folder" or "make new directory." Whichever the option is, create one, and name it "images". Then open that Images folder, which is called a directory, and upload the header image into that folder.
Go back to your site and refresh the page and your header image ought to show up.