Andre,
It would depend upon which layout you are using. Looking at your current website layout, this is how you would do it:
1. Go to the page who's header image you want to change. Do a 'View Source' to see the html. Look at the body tag - it will have an ID. Copy that ID. For this example lets say its your home page. I looked and the ID for your home page is 'pages_index_estatutos'.
2. Look at the HTML and CSS code to figure out where the header image is placed. Looking at your layout, I see that it is a background image of a div with the id of 'header_image'.
3. Next, upload your new header image inside the CSS Editor. For this example, let's say you call it 'header1.jpg'.
4. Add the following code to your CSS Editor Override page:
body#pages_index_estatutos div#header_image{
background: url(header1.jpg) top left no-repeat !important;
}
5. Done!