This is quiet easy. Edit the page you want to modify. Select "Text Layout" from "Format Text" ribbon and set it to "Two columns with header" or another layout having a full width element on top.
Now select the ribbon "Insert" and insert a picture.
Now as you selected or uploaded a picture select the picture ribbon. Make sure the horizontal size is on 100% to fit the SharePoint page at all times. I used a logo with 256x256px which I wanted to stretch horizontaly but not verticaly so I disabled "Lock Aspect Ratio".
Save the page and you got your banner on top fitting all common display resolutions.
For SharePoint 2013, you could follow the steps in this blog with SharePoint Designer:
If you want, You could try using "Content Editor" web part and insert HTML Snippets.
You could do so by add a "Content Editor" web part. Afterwards click the upper right corner of the web part and click "Edit Web Part". Now click "Edit Snippet" and insert your HTML Snippet. click apply and click ok.
I used Sharepoint Designer.
You have to create an html page from the master page. Add the header reference to the html master page. (basically it's the editable file linked to the master page).
<div id="s4-workspace"> <div class="banner"> <img src="https://xxxxxx.sharepoint.com/sites/xx/xxxx/image.png" alt="company banner" width="1000px" height="105px" /> </div> <div id="s4-bodyContainer">
Look for the "s4" divs and place it between them accordingly.
Set up your CSS for the banner then link the .css file in Master Pages under Site Settings.