One common template change is to use your own graphic/image. Simple graphics (not banners) are linked in the html file. Simply change the reference to the image of your choice in the html file of your template. In the adminsitrative interface do this by going to Site =>Template manager and then selecting your template. Click the icon for html.
Keep in mind that it if it is a different size than the original image this may change the appearance of the site in unexpected ways.
Additional information:
The images for a given template are generally located in this folder:
/templates/templatename/images (Substitute the name of the template you are using in place of "templatename").
A trick for finding the name of the image is to put your cursor over it and click right. Select view image. This will display the image and give its full url. Sometimes the images are background images. This is viewable in Firefox or you can look for the background tag in your page source.
How to upload an image:
There are many ways to upload images. Which one you use will depend on your host and server.
1. You can use an ftp client.
2. You can use a cpanel file manager.
3. You can use the media manager.
4. You can use various extensions that allow uploading, including joomlaexplorer and galleries.