In the back-end administrative interface navigate to Extensions -> Template Manager. Select the template you want to modify and click edit. Then click the icon for Edit HTML.
Keep in mind that if the new image 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: <your Joomla home folder>/templates/<your template name>/images.
- A trick for finding the name of the image is to put your cursor over it and right-click the mouse. 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.
- There are different ways to upload images. Which one you use will depend on your host and server.
- the Media manager.
- an FTP client (for example, the free program Filezilla)
- a cPanel file manager.
- various extensions that allow uploading, such as JoomlaXplorer or eXtplorer. A list of the available extensions for this is available in the Administration / File Management section of the Joomla Extensions website.