Hello,
First you need to upload image to wp-content\themes\legenda\images folder. You can use this code in custom.css file to add image into header area.
.header{ background-image: url(images/here_is_name_of_your_image.png); }
Here is a video tutorial how to create custom.css: https://www.youtube.com/watch?v=Qok2zRedRMY&feature=youtu.be.
Regards, Robert Hall.
