Do you want to customize login screen to see custom images that are related to your own company?
No problem, just put it into the Images folder (C:\Program Files (x86)\Acumatica ERP\<Your Site Name>\Icons) with the specific name pattern (login_bg<Number>.jpg).
Note that you can use many different images types, not only jpg. For example, dynamic .gif file can be super nice. Check this link: Cinemagraph