(Tip: The best way to learn how to upload images is to watch the “Tour” on each page (click the “tour” button in the lower right hand corner of any page), and/or to watch the tutorial videos next to each item. They are extremely quick tutorials that will give you the answer you need immediately.)
To change the header image on any page, you have 2 options. First, you can edit the “default header” which is site wide (this is the header that came with your website). When editing any web page, you will see a link to “edit default header” in the Header Image section. Just click on that and it will take you to the page where you can edit it and/or turn it off. Here's a more in depth tutorial on How to Edit the Default Header.
Your second option is to apply a header image to that specific page, and not side wide. You do this by checking the option to override the default header on the specific page you are editing. Again, there are tutorial videos right there to help you.
If you want to add an image into the main text content area of any page, you can upload it using the WSYWIG editor. Again, there is a video button located next to the WSYWIG and we recommend watching it, as it shows you not only how to upload an image, but how to do all sorts of cool stuff to your pages.