Oracle Logo that is displayed on the Login Screen of the PIA can be changed quite easily.
This image is stored in the Webserver Image directory.
On the Webserver
Go to PS_HOME/webserv/domain_name/applications/peoplesoft/PORTAL/ps/images
Find OPSE_logo (for PT 8.48) or ps_logo (for earlier versions) and backup this file. Now paste desired logo image in this folder and rename it as OPSE_log or ps_logo depending on the PeopleTools Version.
Make Sure image dimensions are near in size to these original logos otherwise you wont be able to see any logo.