I will recommend you to visit some *removed tutorial website like http://www.w3schools.com!

Edit the stylesheet.css file for that skin, find:

.header3  {
background:url("rsc/img/headers/vineyards.jpg") 

Delete the vineyards.jpg

Upload your own file into the rsc/img/headers folder. Add the name of the file to where the vineyards.jpg bit went in the above code. Save stylesheet.