If you deal with something like a logo, or a backgroung image (i.e., a static image, not depending on a previous choice):
- click on the page to select it;
- click on Properties, Layout tab;
- click on Show/Edit button of Page Layout property;
- edit the Default.page.template: search the point in the pseudo-HTML code in which you want to show the image and add the img tag, like this:
<img src="smiley.gif" alt="Smiley face" height="48" width="48">
Take care not to break the structure of the Page Layout Template, take care to address correctly the image url, and it will work.
You may then make more flexible this approach by defining a new parameter in the template, so next time (on a different page, for example in which you may want to add a "small-logo" instead of a"big-logo") you will reuse exacly the same template, jusk clicking on the blue/green bullet "Layout Parameters". Lots of examples about how to do it are in several default templates.