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From Joomla! Documentation
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. Do this by, in the administrative interface, going to Site>>Template Manager and then selecting your template. Click the icon for html.
From the administrative interface go to the drop down menu item
- Extensions → Template Manager.
Keep in mind that if it is a different size than the original image this may change the appearance of the site in unexpected ways.
The images for a given template are generally located in this folder:
(where you substitute the name of the template you are using).