You can check this answer out. It seems to have the solution for you:
Steps provided in the link :
To include custom logo, you will have to modify python code. NOTE: this is risky, not supported and will be overwritten upon upgrade.
Step 1. You will need to come up with an SVG for your logo
Step 2. Modify $SPLUNK_HOME/lib/python2.7/site-packages/splunk/pdf/pdfrenderer.py file. At the end of the file, change the variable assignment of _splunkLogoSvg to your new logo.
Step 3. Restart Splunk and generate the PDF with the new logo design.
Thanks David But in answers state that for scheduled reports you can just remove the splunk logo. We want to add custom logo.
Have any one tested this. Thanks
If you are checking this question at 2018 like me 🙂 then this can be done easily, Settings -> Server settings -> Email settings -> PDF Report Settings