How to add banner

Hi. I am very new in wordpress and in advance sorry if my questions are simple. my creating site https://akcijoskuponas.lt. I want to add banner in the top of page. I have created image (jpeg) and would like to add it. Where do I have to load it? I am reading image upload wordpress but I don’t find answer.
And other question: why there is no coupon code when on the coupon push “get code” button? I wrote it in coupon management field code.

Thank you in advance for the help.