Images may be inserted several ways but I recommend uploading them to the "Attachments" page first and then changing the url to each image in the table. This method allows you to upload images to one place, the "Attachments" page, then use them in any page.
When images are added, they will be automatically re-sized to the current size in this page.
Template images are located in the "Attachments" page. To replace them with your own , select the check box next to the image and the "Replace" button. A dialog box will open to your computer. Select an image and wait while it is uploaded.