While a Joomla website is offline it might be nice to customize the default offline page to include a company logo and custom message.
The file templates/system/offline.php contains the markup that will be displayed when a Joomla site is offline.
If there is a file named offline.php in your template folder it will override the default offline file templates/system/offline.php.
Copy templates/system/offline.php to your template folder:
Then customize the copied file.
The offline message can be changed in Site > Global Configuration.
Custom Offline Image
Near the top of offline.php there is this code:
<img src="/images/joomla_logo_black.jpg" alt="Joomla! Logo" align="middle" />
This could be edited to something like:
<img src="/templates/template-name/images/your-image.jpg" alt="Your Alt Text" align="middle" />