Please look at the method INIT_DEFAULT_PROPERTIES of /UI2/CL_SRA_LOGIN class. set_property( iv_name = 'img_logo' iv_value = '/sap/public/bc/ui2/logon/img/sap_logo.png' ) Easy way is just replacing the images in the MIME repository.
Take a look at Customizing Fiori Logon Page thread
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