Hi,
EPESI is released under MIT license: http://www.epesi.org/License
You can modify the login screen and add your own logo but the following can not be removed:
- License terms (MIT)
- Copyright by Telaxus LLC notice (part of MIT licensing terms)
- logo and link: powered by EPESI

Also - in the application's menu the link Support -> About must be visible as it shows license terms and credits - including all 3rd party software and libraries used in EPESI:

The proper way to show your credits would be to say:
If you modified or changed a code in a significant way:
Copyright by YourCompanyName
Copyright by Telaxus LLC 2013
EPESI powered (logo)
If you did not modify or changed EPESI code but just offer it as Saas service:
Hosting by YourCompanyName (example)
Copyright by Telaxus LLC 2013
EPESI powered (logo)
See this post: viewtopic.php?f=2&t=1640&p=5714
If you have further questions please respond to this post or send me PM.