Use note: 205487 User-defined text on SAPGui logon screen to add text to the default SAPGui logon screen.

Steps:

1. Go to tcode SE61
2. Select general text
3. Create text with name ZLOGON_SCREEN_INFO in your language

Text is limited to 16 lines and 60 columns

Also, You can incorporate graphic icons by inserting icon codes in the text.
Eg.: The following code: @1D@ will insert the STOP ICON on the page.

You can get a list of codes and icons by running report “RSTXICON”