http://docs.splunk.com/Documentation/Splunk/6.5.2/AdvancedDev/CustomizeLogin
Steps
Log into the Splunk instance and navigate to Settings > System > Server Settings > Login Background.
Select one of the following options.
Background option Description
Custom image To use a custom background, upload the image file and click Choose.
Default image Use the default background image.
No image Do not display an image on the login page.
Use the Preview screen to preview the login page customization.
Click Save.
Restart the Splunk instance to view the changes.
