When you are logged into the site you should see an edit link near the title of the content instance, ie next to the word "Welcome" in the default new site content. Then in the editor you can delete those images by clicking on the image and backspace. To add a new image put the cursor in the editor where you would like the new image to be, click the Image toolbar item in the editor (mouse over the toolbar items to see their titles). The image dialog opens then you click the button to browse the server which opens another dialog page where you can upload an image or select existing images on the server.
See also Training Videos to learn how to use mojoPortal
Note that I moved this thread from the developer forum to the questions about site administration forum, since it has nothing to do with software development.
Hope that helps,
Joe