How do you change the image(s) in your template?
From Joomla! Documentation
One common template change is to use your own graphics and images. 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 going to Administrator → Site → Template Manager and selecting your template. Click the icon for HTML.
From the administrative interface go to the drop down menu item
- Extensions → Template Manager.
If it is a different size than the original image, the appearance of the site might change.
The images for a given template are generally located in the /templates/templatename/images folder. (Substitute the name of the template you are using.)