Please refer below steps from documents:
**Add a custom logo**
If you are using Splunk Enterprise, you can customize the login page logo.
**Prerequisites**
The maximum image size is 485px wide and 100px high. If the image exceeds these limits, the image is automatically resized.
Review the loginCustomLogo setting details in the web.conf spec file.
**Steps**
(Optional) If you are using an image file, put it into the following directory location.
$SPLUNK_HOME/etc/apps/<app_name>/appserver/static/logo
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